dm: use dm_target_offset macro

Use new dm_target_offset() macro to avoid most references to ti->begin
in dm targets.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 6401bfa..368e8e9 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -1284,7 +1284,7 @@
 		return DM_MAPIO_REMAPPED;
 	}
 
-	io = crypt_io_alloc(ti, bio, bio->bi_sector - ti->begin);
+	io = crypt_io_alloc(ti, bio, dm_target_offset(ti, bio->bi_sector));
 
 	if (bio_data_dir(io->base_bio) == READ)
 		kcryptd_queue_io(io);
@@ -1406,7 +1406,7 @@
 		return max_size;
 
 	bvm->bi_bdev = cc->dev->bdev;
-	bvm->bi_sector = cc->start + bvm->bi_sector - ti->begin;
+	bvm->bi_sector = cc->start + dm_target_offset(ti, bvm->bi_sector);
 
 	return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
 }