block: add queue_is_mq() helper

Various spots check for q->mq_ops being non-NULL, but provide
a helper to do this instead.

Where the ->mq_ops != NULL check is redundant, remove it.

Since mq == rq-based now that legacy is gone, get rid of the
queue_is_rq_based() and just use queue_is_mq() everywhere.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/bsg.c b/block/bsg.c
index 9a442c2..44f6028 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -471,7 +471,7 @@ int bsg_register_queue(struct request_queue *q, struct device *parent,
 	/*
 	 * we need a proper transport to send commands, not a stacked device
 	 */
-	if (!queue_is_rq_based(q))
+	if (!queue_is_mq(q))
 		return 0;
 
 	bcd = &q->bsg_dev;