dm: use clone in map_request function

This patch changes the argument of map_request() to clone request
from original request.  No functional change.

This patch is a preparation for PATCH 9, which needs to use
map_request() for clones sharing an original barrier request.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -1495,11 +1495,10 @@ static int dm_prep_fn(struct request_queue *q, struct request *rq)
 	return BLKPREP_OK;
 }
 
-static void map_request(struct dm_target *ti, struct request *rq,
+static void map_request(struct dm_target *ti, struct request *clone,
 			struct mapped_device *md)
 {
 	int r;
-	struct request *clone = rq->special;
 	struct dm_rq_target_io *tio = clone->end_io_data;
 
 	/*
@@ -1576,7 +1575,7 @@ static void dm_request_fn(struct request_queue *q)
 
 		blk_start_request(rq);
 		spin_unlock(q->queue_lock);
-		map_request(ti, rq, md);
+		map_request(ti, rq->special, md);
 		spin_lock_irq(q->queue_lock);
 	}