cgroup: update comment about dropping cgroup kn refs

the cgroup is actually freed in css_free_rwork_fn() now
the ref count of the cgroup's kernfs_node is also dropped there
so we need to update the corresponding comment in cgroup_mkdir()

Signed-off-by: Haorui He <mail@hehaorui.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: hehaorui

kernel/cgroup/cgroup.c

@@ -5835,7 +5835,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 	}
 
 	/*
-	 * This extra ref will be put in cgroup_free_fn() and guarantees
+	 * This extra ref will be put in css_free_rwork_fn() and guarantees
 	 * that @cgrp->kn is always accessible.
 	 */
 	kernfs_get(cgrp->kn);