cgroup: move number_of_cgroups test out of rebind_subsystems() into cgroup_remount()

rebind_subsystems() currently fails if the hierarchy has any !root
cgroups; however, on the planned unified hierarchy,
rebind_subsystems() will be used while populated.  Move the test to
cgroup_remount(), which is the only place the test is necessary
anyway.

As it's impossible for the other two callers of rebind_subsystems() to
have populated hierarchy, this doesn't make any behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -1021,13 +1021,6 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 		}
 	}
 
-	/* Currently we don't handle adding/removing subsystems when
-	 * any child cgroups exist. This is theoretically supportable
-	 * but involves complex error handling, so it's being left until
-	 * later */
-	if (root->number_of_cgroups > 1)
-		return -EBUSY;
-
 	ret = cgroup_populate_dir(cgrp, added_mask);
 	if (ret)
 		return ret;
@@ -1373,6 +1366,12 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 		goto out_unlock;
 	}
 
+	/* remounting is not allowed for populated hierarchies */
+	if (root->number_of_cgroups > 1) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+
 	ret = rebind_subsystems(root, added_mask, removed_mask);
 	if (ret)
 		goto out_unlock;