blk-mq: use plug for devices that implement ->commits_rqs()

If we have that hook, we know the driver handles bd->last == true in
a smart fashion. If it does, even for multiple hardware queues, it's
a good idea to flush batches of requests to the device, if we have
batches of requests from the submitter.
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1945,7 +1945,11 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		/* bypass scheduler for flush rq */
 		blk_insert_flush(rq);
 		blk_mq_run_hw_queue(data.hctx, true);
-	} else if (plug && q->nr_hw_queues == 1) {
+	} else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs)) {
+		/*
+		 * Use plugging if we have a ->commit_rqs() hook as well, as
+		 * we know the driver uses bd->last in a smart fashion.
+		 */
 		unsigned int request_count = plug->rq_count;
 		struct request *last = NULL;