daemon.cleanupContainer: nullify container RWLayer upon release
daemon.cleanupContainer: nullify container RWLayer upon release

ReleaseRWLayer can and should only be called once (unless it returns
an error), but might be called twice in case of a failure from
`system.EnsureRemoveAll(container.Root)`. This results in the
following error:

> Error response from daemon: driver "XXX" failed to remove root filesystem for YYY: layer not retained

The obvious fix is to set container.RWLayer to nil as soon as
ReleaseRWLayer() succeeds.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

rancher/docker,ibuildthecloud/docker,yongtang/docker,mahak/docker,tmp6154/docker,mahak/docker,yongtang/docker,estesp/docker,dccurtis/docker,KarthikNayak/docker,vdemeester/docker,dmcgowan/docker,noironetworks/docker,crosbymichael/docker,tophj-ibm/moby,yongtang/docker,estesp/docker,cpuguy83/docker,jvanz/docker,KingEmet/docker,jlhawn/docker,Mokolea/docker,AkihiroSuda/docker,whuwxl/docker,jimjag/docker,dmcgowan/docker,bearstech/moby,maxamillion/docker,moul/docker,SvenDowideit/docker,jameszhan/docker,moul/docker,x1022as/docker,trebonian/docker,x1022as/docker,yosifkit/docker,justincormack/docker,jwhonce/docker,laijs/moby,yosifkit/docker,thaJeztah/docker,crosbymichael/docker,paralin/docker,duglin/docker,KarthikNayak/docker,allencloud/docker,dims/docker,x1022as/docker,johnstep/moby-moby,AkihiroSuda/docker,nawawi/docker,jlhawn/docker,noironetworks/docker,AkihiroSuda/docker,moul/docker,dccurtis/docker,YuPengZTE/docker,SvenDowideit/docker,flavio/docker,rremer/moby,dccurtis/docker,tophj-ibm/docker,moby/moby,rancher/docker,laijs/moby,nawawi/docker,pradipd/moby,nawawi/docker,ibuildthecloud/docker,KingEmet/docker,kojiromike/docker,mikebrow/docker,allencloud/docker,vdemeester/docker,nalind/docker,konstruktoid/docker-upstream,nalind/docker,flavio/docker,laijs/docker,moby/moby,trebonian/docker,pradipd/moby,flavio/docker,flavio/docker,Mokolea/docker,Ramzec/docker,mahak/docker,maxamillion/docker,AkihiroSuda/docker,nawawi/docker,justincormack/docker,trebonian/docker,konstruktoid/docker-1,johnstep/moby-moby,ibuildthecloud/docker,whuwxl/docker,djs55/docker,paralin/docker,duglin/docker,kolyshkin/docker,cpuguy83/docker,crosbymichael/docker,jthelin/docker,noironetworks/docker,yosifkit/docker,moby/moby,mahak/docker,tonistiigi/docker,Mokolea/docker,scotttlin/docker,johnstep/moby-moby,maxamillion/docker,laijs/docker,jthelin/docker,paralin/docker,thaJeztah/docker,cpuguy83/docker,nalind/docker,kolyshkin/moby,tmp6154/docker,djs55/docker,vdemeester/docker,bearstech/moby,docker/docker,estesp/docker,unclejack/docker,jvanz/docker,kojiromike/docker,docker/docker,pradipd/moby,konstruktoid/docker-1,mikebrow/docker,unclejack/docker,gnailuy/docker,tophj-ibm/docker,djs55/docker,moul/docker,tmp6154/docker,SvenDowideit/docker,docker/docker,thaJeztah/docker,jameszhan/docker,Ramzec/docker,bearstech/moby,KarthikNayak/docker,kolyshkin/moby,allencloud/docker,kolyshkin/moby,unclejack/docker,tophj-ibm/moby,YuPengZTE/docker,dims/docker,konstruktoid/docker-upstream,laijs/moby,cpuguy83/docker,dmcgowan/docker,Ramzec/docker,jimjag/docker,jimjag/docker,jlhawn/docker,gnailuy/docker,tonistiigi/docker,bearstech/moby,scotttlin/docker,Mokolea/docker,laijs/docker,scotttlin/docker,konstruktoid/docker-1,ibuildthecloud/docker,jvanz/docker,ibuildthecloud/docker,tonistiigi/docker,justincormack/docker,tophj-ibm/moby,gnailuy/docker,jwhonce/docker,gnailuy/docker,jwhonce/docker,rremer/moby,rancher/docker,kojiromike/docker,dims/docker,nalind/docker,trebonian/docker,Ramzec/docker,konstruktoid/docker-upstream,YuPengZTE/docker,thaJeztah/docker,whuwxl/docker,paralin/docker,KingEmet/docker,tophj-ibm/docker,mikebrow/docker,kolyshkin/docker,jimjag/docker,jameszhan/docker,duglin/docker,konstruktoid/docker-upstream,moby/moby,yosifkit/docker,rremer/moby