Revert "cgroup: avoid attaching a cgroup root to two different superblocks"

This reverts commit bfb0b80d

.

Andrei reports CRIU test hangs with the patch applied.  The bug fixed
by the patch isn't too likely to trigger in actual uses.  Revert the
patch for now.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Link: http://lkml.kernel.org/r/20170414232737.GC20350@outlook.office365.com