block: remove q->prepare_flush_fn completely

This removes q->prepare_flush_fn completely (changes the
blk_queue_ordered API).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 00c8105..d505a96 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1901,7 +1901,7 @@
 	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, NULL);
+	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH);
 
 	md->disk = alloc_disk(1);
 	if (!md->disk)