dm: clear bi_end_io on remapping failure

As a precaution, set bi_end_io to NULL when failing to remap.
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -1016,6 +1016,7 @@ static void __map_bio(struct dm_target *ti, struct bio *clone,
 		/*
 		 * Store bio_set for cleanup.
 		 */
+		clone->bi_end_io = NULL;
 		clone->bi_private = md->bs;
 		bio_put(clone);
 		free_tio(md, tio);