dm-buffered: fix calculations for prefetch

Fix the calculation of "start" and "blocks" when calling
dm_bufio_prefetch.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-buffered-target.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6/drivers/md/dm-buffered-target.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-buffered-target.c
+++ linux-2.6/drivers/md/dm-buffered-target.c
@@ -272,7 +272,7 @@ static void _process_bio(struct work_str
 	struct buffered_c *bc = container_of(work, struct buffered_c, buffered_ws);
 	struct bio *bio;
 	bool write, queue;
-	sector_t blocks, start, end;
+	sector_t blocks, sectors, start, end;
 
 	spin_lock(&bc->lock);
 	bio = bio_list_pop(&bc->bios);
@@ -287,8 +287,11 @@ static void _process_bio(struct work_str
 	/* Prefetch read and partial write buffers */
 	switch (bio_op(bio)) {
 	case REQ_OP_READ:
-		blocks = _to_block(bc, bio_end_sector(bio) - bio->bi_iter.bi_sector);
-		dm_bufio_prefetch(bc->bufio, start, blocks ?: 1);
+		sectors = bio_end_sector(bio) - (start << bc->block_shift);
+		blocks = _to_block(bc, sectors);
+		if (_sector_mod(bc, sectors))
+			blocks++;
+		dm_bufio_prefetch(bc->bufio, start, blocks);
 		break;
 	case REQ_OP_WRITE:
 		write = true;