cgroup,rcu: convert call_rcu(__free_css_id_cb) to kfree_rcu()

The rcu callback __free_css_id_cb() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(__free_css_id_cb).

Signed-off-by: Lai Jiangshan <[e-mail]>
Acked-by: Paul Menage <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>

kernel/cgroup.c

···
 	return ret;
 }
 
-static void __free_css_id_cb(struct rcu_head *head)
-{
-	struct css_id *id;
-
-	id = container_of(head, struct css_id, rcu_head);
-	kfree(id);
-}
-
 void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
 {
 	struct css_id *id = css->id;
···
 	spin_lock(&ss->id_lock);
 	idr_remove(&ss->idr, id->id);
 	spin_unlock(&ss->id_lock);
-	call_rcu(&id->rcu_head, __free_css_id_cb);
+	kfree_rcu(id, rcu_head);
 }
 EXPORT_SYMBOL_GPL(free_css_id);