Nuke when graphdir is a mount
Nuke when graphdir is a mount

Previously, when the graphdir was a root of a mountpoint, we would not
be able to cleanup the graphdir, as the script would umount in case
target dir is a mount itself

```/etc/mtab
/dev/xvdc1 /var/lib/docker btrfs
```

When running the script, the graphdir would be unmounted and it would
remove a (possibly) empty folder.

```
Nuking /var/lib/docker ...
  (if this is wrong, press Ctrl+C NOW!)

  + sleep 10

  + umount -f /var/lib/docker
  + rm -rf /var/lib/docker
  ```

This PR includes the necessary changes to nuke the folder on this
scenario, including when the graphdir is a btrfs mount iself.

Signed-off-by: Bruno Tavares <bfe24634c697f21aa386747beee78fb12cf01175@thoughtworks.com>

nawawi/docker,PatrickLang/moby,Mokolea/docker,Syntaxide/docker,rhatdan/docker,liaoqingwei/docker,q384566678/docker,resin-io/docker,thtanaka/docker,charleszheng44/moby,JMesser81/docker,jimjag/docker,tonistiigi/docker,bonczj/moby-json-file-logger,lixiaobing10051267/docker,mikebrow/docker,tophj-ibm/moby,noironetworks/docker,shishir-a412ed/docker,madhanrm/docker,darrenstahlmsft/moby,HideoYamauchi/docker,coreos/docker,darrenstahlmsft/docker,yongtang/docker,tonistiigi/docker,moby/moby,tklauser/docker,xuxinkun/docker,ColinHebert/docker,naveed-jamil-tenpearls/moby,catinthesky/docker,konstruktoid/docker-upstream,madhanrm/docker,stevvooe/docker,jvanz/docker,duglin/docker,Mokolea/docker,HideoYamauchi/docker,qudongfang/docker,akira-baruah/docker,yosifkit/docker,mixja/docker,coreos/docker,trebonian/docker,lixiaobing10051267/docker,qudongfang/docker,dgageot/docker,naveed-jamil-tenpearls/moby,sarnautov/moby,tklauser/docker,thaJeztah/docker,PatrickLang/moby,bearstech/moby,yosifkit/docker,kojiromike/docker,lixiaobing1/docker,shhsu/docker,kolyshkin/moby,moby/moby,mixja/docker,mstanleyjones/docker,rancher/docker,noironetworks/docker,erikstmartin/docker,msabansal/docker,mstanleyjones/docker,nawawi/docker,laijs/moby,jthelin/docker,POLJM242/docker,xuxinkun/docker,mike1729/docker,jameszhan/docker,jimjag/docker,ripcurld0/moby,scotttlin/docker,allencloud/docker,yosifkit/docker,Ramzec/docker,moul/docker,sdurrheimer/docker,imkin/moby,vdemeester/docker,erikh/docker,flavio/docker,dims/docker,michael-holzheu/docker,dperny/docker,michael-holzheu/docker,fcrisciani/docker,resin-io/docker,ripcurld0/moby,tianon/docker,konstruktoid/docker-1,djs55/docker,KingEmet/docker,AkihiroSuda/docker,akira-baruah/docker,jwhonce/docker,bonczj/moby-json-file-logger,qudongfang/docker,mtrmac/docker,gschaetz/moby,srust/docker,moul/docker,tmp6154/docker,haveatry/docker,maxamillion/docker,tophj-ibm/docker,coolljt0725/docker,thtanaka/docker,cpuguy83/docker,KarthikNayak/docker,rhatdan/docker,raja-sami-10p/moby,tophj-ibm/docker,crosbymichael/docker,kencochrane/docker,resin-io/docker,raja-sami-10p/moby,crosbymichael/docker,konstruktoid/docker-upstream,scotttlin/docker,gschaetz/moby,haveatry/docker,lixiaobing1/docker,thaJeztah/docker,msabansal/docker,cyphar/docker,jimjag/docker,dims/docker,YuPengZTE/docker,gnailuy/docker,stevvooe/docker,darrenstahlmsft/moby,ibuildthecloud/docker,coreos/docker,rancher/docker,tklauser/docker,yongtang/docker,flavio/docker,mtrmac/docker,dccurtis/docker,cpuguy83/docker,darrenstahlmsft/docker,justincormack/docker,moypray/docker,djs55/docker,dims/docker,mkumatag/docker,rhatdan/docker,ffoysal/docker,kolyshkin/moby,vdemeester/docker,shishir-a412ed/docker,docker/docker,miminar/docker,konstruktoid/docker-upstream,mike1729/docker,kojiromike/docker,sterchelen/docker,dperny/docker,danyalKhaliq10p/moby,ColinHebert/docker,cyphar/docker,kencochrane/docker,YuPengZTE/docker,jimjag/docker,Ramzec/docker,amatsus/docker,miminar/docker,ndeloof/docker,danyalKhaliq10p/moby,allencloud/docker,darrenstahlmsft/moby,raja-sami-10p/moby,hqhq/moby,amatsus/docker,thtanaka/docker,laijs/moby,jvanz/docker,Syntaxide/docker,mtesselH/docker,dmcgowan/docker,nawawi/docker,moul/docker,alfred-landrum/docker,danyalKhaliq10p/moby,michael-holzheu/docker,erikstmartin/docker,KingEmet/docker,pradipd/moby,naveed-jamil-tenpearls/moby,thaJeztah/docker,Ramzec/docker,AkihiroSuda/docker,sterchelen/docker,jlhawn/docker,estesp/docker,rremer/moby,moby/moby,prertik/docker,crosbymichael/docker,erikh/docker,catinthesky/docker,YuPengZTE/docker,tophj-ibm/moby,sdurrheimer/docker,Mokolea/docker,cyphar/docker,laijs/moby,prertik/docker,mike1729/docker,JMesser81/docker,pradipd/moby,KarthikNayak/docker,yongtang/docker,docker/docker,bearstech/moby,q384566678/docker,moypray/docker,yummypeng/docker,KarthikNayak/docker,ibuildthecloud/docker,SvenDowideit/docker,yasiramin10p/moby,allencloud/docker,dgageot/docker,mkumatag/docker,diogomonica/docker,johnstep/moby-moby,justincormack/docker,harche/docker,duglin/docker,estesp/docker,rremer/moby,ndeloof/docker,maxamillion/docker,yummypeng/docker,mtesselH/docker,coolljt0725/docker,maxamillion/docker,SvenDowideit/docker,shhsu/docker,thaJeztah/docker,Syntaxide/docker,pramodhkp/moby,amatsus/docker,bearstech/moby,diogomonica/docker,ibuildthecloud/docker,paralin/docker,gnailuy/docker,PatrickLang/moby,pramodhkp/moby,rancher/docker,alfred-landrum/docker,ffoysal/docker,mixja/docker,dmcgowan/docker,cpuguy83/docker,ndeloof/docker,dperny/docker,mikebrow/docker,mtesselH/docker,gjaskiewicz/moby,duglin/docker,jameszhan/docker,madhanrm/docker,pradipd/moby,nalind/docker,yummypeng/docker,Ramzec/docker,ffoysal/docker,mahak/docker,shhsu/docker,gjaskiewicz/moby,laijs/docker,mikebrow/docker,AkihiroSuda/docker,noironetworks/docker,paralin/docker,tmp6154/docker,tophj-ibm/moby,trebonian/docker,nalind/docker,catinthesky/docker,laijs/docker,mikedougherty/docker,bearstech/moby,charleszheng44/moby,gnailuy/docker,imkin/moby,kolyshkin/docker,darrenstahlmsft/docker,djs55/docker,KingEmet/docker,harche/docker,whuwxl/docker,nawawi/docker,x1022as/docker,johnstep/moby-moby,Mokolea/docker,trebonian/docker,tianon/docker,jvanz/docker,alfred-landrum/docker,jlhawn/docker,mkumatag/docker,jwhonce/docker,yasiramin10p/moby,docker/docker,sterchelen/docker,estesp/docker,laijs/docker,lixiaobing1/docker,moul/docker,liaoqingwei/docker,nalind/docker,dccurtis/docker,msabansal/docker,shishir-a412ed/docker,q384566678/docker,jameszhan/docker,lixiaobing10051267/docker,unclejack/docker,x1022as/docker,konstruktoid/docker-1,whuwxl/docker,SvenDowideit/docker,jlhawn/docker,dsheets/moby,stevvooe/docker,unclejack/docker,mahak/docker,erikstmartin/docker,tophj-ibm/docker,xuxinkun/docker,kolyshkin/docker,HideoYamauchi/docker,moby/moby,mahak/docker,harche/docker,konstruktoid/docker-1,kencochrane/docker,flavio/docker,unclejack/docker,srust/docker,dgageot/docker,gschaetz/moby,trebonian/docker,kojiromike/docker,pramodhkp/moby,sarnautov/moby,mahak/docker,diogomonica/docker,yosifkit/docker,miminar/docker,ColinHebert/docker,sarnautov/moby,tonistiigi/docker,rremer/moby,gjaskiewicz/moby,POLJM242/docker,mikedougherty/docker,dmcgowan/docker,hqhq/moby,bonczj/moby-json-file-logger,jthelin/docker,jwhonce/docker,ibuildthecloud/docker,whuwxl/docker,vdemeester/docker,JMesser81/docker,mikedougherty/docker,flavio/docker,mstanleyjones/docker,AkihiroSuda/docker,gnailuy/docker,fcrisciani/docker,kolyshkin/moby,mtrmac/docker,prertik/docker,erikh/docker,fcrisciani/docker,dsheets/moby,scotttlin/docker,haveatry/docker,paralin/docker,charleszheng44/moby,justincormack/docker,imkin/moby,johnstep/moby-moby,sdurrheimer/docker,liaoqingwei/docker,dsheets/moby,moypray/docker,x1022as/docker,nalind/docker,yasiramin10p/moby,dccurtis/docker,cpuguy83/docker,ripcurld0/moby,coolljt0725/docker,paralin/docker,srust/docker,konstruktoid/docker-upstream,tmp6154/docker,ibuildthecloud/docker,hqhq/moby