block: move poll code to blk-mq

The poll code is blk-mq specific, let's move it to blk-mq.c. This
is a prep patch for improving the polling code.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/io-cmd.c

@@ -96,7 +96,7 @@ static void nvmet_execute_rw(struct nvmet_req *req)
 
 	cookie = submit_bio(bio);
 
-	blk_poll(bdev_get_queue(req->ns->bdev), cookie);
+	blk_mq_poll(bdev_get_queue(req->ns->bdev), cookie);
 }
 
 static void nvmet_execute_flush(struct nvmet_req *req)