cgroup: remove the restriction of subset-of-root-set
cgroup: remove the restriction of subset-of-root-set

At one point in the cgns patchsets I had removed this, but somehow it got
lost in the shuffle. Since we support this now, let's remove this
restriction.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

AuthenticEshkinKot/criu,ldu4/criu,AuthenticEshkinKot/criu,ldu4/criu,ldu4/criu,KKoukiou/criu-remote,tych0/criu,AuthenticEshkinKot/criu,efiop/criu,efiop/criu,eabatalov/criu,eabatalov/criu,tych0/criu,efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,eabatalov/criu,ldu4/criu,ldu4/criu,KKoukiou/criu-remote,efiop/criu,ldu4/criu,eabatalov/criu,tych0/criu,AuthenticEshkinKot/criu,tych0/criu,tych0/criu,KKoukiou/criu-remote,efiop/criu,AuthenticEshkinKot/criu,eabatalov/criu,eabatalov/criu,tych0/criu,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote