block: add a poll_fn callback to struct request_queue

That we we can also poll non blk-mq queues.  Mostly needed for
the NVMe multipath code, but could also be useful elsewhere.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/direct-io.c b/fs/direct-io.c
index 62cf812..d2bc339 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -486,7 +486,7 @@
 		dio->waiter = current;
 		spin_unlock_irqrestore(&dio->bio_lock, flags);
 		if (!(dio->iocb->ki_flags & IOCB_HIPRI) ||
-		    !blk_mq_poll(dio->bio_disk->queue, dio->bio_cookie))
+		    !blk_poll(dio->bio_disk->queue, dio->bio_cookie))
 			io_schedule();
 		/* wake up sets us TASK_RUNNING */
 		spin_lock_irqsave(&dio->bio_lock, flags);