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

The rcu callback free_cgroup_rcu() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(free_cgroup_rcu).
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/cgroup.c

@@ -806,13 +806,6 @@ static int cgroup_call_pre_destroy(struct cgroup *cgrp)
 	return ret;
 }
 
-static void free_cgroup_rcu(struct rcu_head *obj)
-{
-	struct cgroup *cgrp = container_of(obj, struct cgroup, rcu_head);
-
-	kfree(cgrp);
-}
-
 static void cgroup_diput(struct dentry *dentry, struct inode *inode)
 {
 	/* is dentry a directory ? if so, kfree() associated cgroup */
@@ -850,7 +843,7 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
 		 */
 		BUG_ON(!list_empty(&cgrp->pidlists));
 
-		call_rcu(&cgrp->rcu_head, free_cgroup_rcu);
+		kfree_rcu(cgrp, rcu_head);
 	}
 	iput(inode);
 }