vmalloc,rcu: Convert call_rcu(rcu_free_vb) to kfree_rcu()

The rcu callback rcu_free_vb() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(rcu_free_vb).
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

mm/vmalloc.c

@@ -830,13 +830,6 @@ static struct vmap_block *new_vmap_block(gfp_t gfp_mask)
 	return vb;
 }
 
-static void rcu_free_vb(struct rcu_head *head)
-{
-	struct vmap_block *vb = container_of(head, struct vmap_block, rcu_head);
-
-	kfree(vb);
-}
-
 static void free_vmap_block(struct vmap_block *vb)
 {
 	struct vmap_block *tmp;
@@ -849,7 +842,7 @@ static void free_vmap_block(struct vmap_block *vb)
 	BUG_ON(tmp != vb);
 
 	free_vmap_area_noflush(vb->va);
-	call_rcu(&vb->rcu_head, rcu_free_vb);
+	kfree_rcu(vb, rcu_head);
 }
 
 static void purge_fragmented_blocks(int cpu)