dm: remove check that prevents mapping empty bios

Remove the check that the size of the cloned bio is not zero because a
subsequent patch needs to send zero-sized barriers down this path.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 8498dc4..7d9ca70 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -637,11 +637,6 @@
 	sector_t sector;
 	struct mapped_device *md;
 
-	/*
-	 * Sanity checks.
-	 */
-	BUG_ON(!clone->bi_size);
-
 	clone->bi_end_io = clone_endio;
 	clone->bi_private = tio;