cgroup: avoid attaching a cgroup root to two different superblocks, take 2

cgroup: avoid attaching a cgroup root to two different superblocks, take 2

Commit bfb0b80db5f9 ("cgroup: avoid attaching a cgroup root to two
different superblocks") is broken.  Now we try to fix the race by
delaying the initialization of cgroup root refcnt until a superblock
has been allocated.

Reported-by: Dmitry Vyukov <[email protected]>
Reported-by: Andrei Vagin <[email protected]>
Tested-by: Andrei Vagin <[email protected]>
Signed-off-by: Zefan Li <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>