block: use helper function to test queue register

We have defined common interface blk_queue_registered() to
test QUEUE_FLAG_REGISTERED. Just use it.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/elevator.c b/block/elevator.c
index b506895..431a2a1 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -672,7 +672,7 @@ void elevator_init_mq(struct request_queue *q)
 	if (!elv_support_iosched(q))
 		return;
 
-	WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
+	WARN_ON_ONCE(blk_queue_registered(q));
 
 	if (unlikely(q->elevator))
 		return;