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

This reverts commit bfb0b80.

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 <[email protected]>
Reported-by: Andrei Vagin <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

Author: htejun

kernel/cgroup/cgroup-v1.c

@@ -1146,7 +1146,7 @@ struct dentry *cgroup1_mount(struct file_system_type *fs_type, int flags,
 		 * path is super cold.  Let's just sleep a bit and retry.
 		 */
 		pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
-		if (IS_ERR_OR_NULL(pinned_sb) ||
+		if (IS_ERR(pinned_sb) ||
 		    !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
 			mutex_unlock(&cgroup_mutex);
 			if (!IS_ERR_OR_NULL(pinned_sb))