overlay2: doesSupportNativeDiff: add fast path for userns
overlay2: doesSupportNativeDiff: add fast path for userns

When running in userns, returns error (i.e. "use naive, not native")
immediately.

No substantial change to the logic.

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

jameszhan/docker,nalind/docker,AkihiroSuda/docker,djs55/docker,kojiromike/docker,cpuguy83/docker,cpuguy83/docker,yosifkit/docker,mahak/docker,Ramzec/docker,jthelin/docker,vdemeester/docker,moby/moby,flavio/docker,cpuguy83/docker,jimjag/docker,Ramzec/docker,jimjag/docker,thaJeztah/docker,vdemeester/docker,trebonian/docker,jthelin/docker,Ramzec/docker,vdemeester/docker,nalind/docker,docker/docker,gnailuy/docker,jimjag/docker,flavio/docker,moul/docker,mahak/docker,nawawi/docker,djs55/docker,docker/docker,gnailuy/docker,paralin/docker,jimjag/docker,Mokolea/docker,nalind/docker,kojiromike/docker,djs55/docker,gnailuy/docker,flavio/docker,yosifkit/docker,mahak/docker,cpuguy83/docker,nawawi/docker,mahak/docker,konstruktoid/docker-upstream,kojiromike/docker,Mokolea/docker,flavio/docker,thaJeztah/docker,Mokolea/docker,nawawi/docker,nawawi/docker,moby/moby,paralin/docker,trebonian/docker,thaJeztah/docker,konstruktoid/docker-upstream,moby/moby,nalind/docker,thaJeztah/docker,jameszhan/docker,AkihiroSuda/docker,yosifkit/docker,jameszhan/docker,moul/docker,bearstech/moby,moul/docker,konstruktoid/docker-upstream,gnailuy/docker,AkihiroSuda/docker,konstruktoid/docker-upstream,moby/moby,bearstech/moby,Mokolea/docker,AkihiroSuda/docker,paralin/docker,yosifkit/docker,Ramzec/docker,bearstech/moby,docker/docker,paralin/docker,trebonian/docker,bearstech/moby,trebonian/docker,moul/docker