dm: stop using q->prepare_flush_fn

use REQ_FLUSH flag instead.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Alasdair G Kergon <agk@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/md/dm.c

@@ -1455,20 +1455,9 @@ static int dm_request(struct request_queue *q, struct bio *bio)
 	return _dm_request(q, bio);
 }
 
-/*
- * Mark this request as flush request, so that dm_request_fn() can
- * recognize.
- */
-static void dm_rq_prepare_flush(struct request_queue *q, struct request *rq)
-{
-	rq->cmd_type = REQ_TYPE_LINUX_BLOCK;
-	rq->cmd[0] = REQ_LB_OP_FLUSH;
-}
-
 static bool dm_rq_is_flush_request(struct request *rq)
 {
-	if (rq->cmd_type == REQ_TYPE_LINUX_BLOCK &&
-	    rq->cmd[0] == REQ_LB_OP_FLUSH)
+	if (rq->cmd_flags & REQ_FLUSH)
 		return true;
 	else
 		return false;
@@ -1912,8 +1901,7 @@ static struct mapped_device *alloc_dev(int minor)
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 	blk_queue_lld_busy(md->queue, dm_lld_busy);
-	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH,
-			  dm_rq_prepare_flush);
+	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH, NULL);
 
 	md->disk = alloc_disk(1);
 	if (!md->disk)