block: add helper for checking if queue is registered

There are 4 users which check if queue is registered, so add one helper
to check it.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/elevator.c b/block/elevator.c
index 33c15fb5..03d9231 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -656,7 +656,7 @@ static int __elevator_change(struct request_queue *q, const char *name)
 	struct elevator_type *e;
 
 	/* Make sure queue is not in the middle of being removed */
-	if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
+	if (!blk_queue_registered(q))
 		return -ENOENT;
 
 	/*