dm: remove impossible BUG_ON in __send_empty_flush

The flush_bio in question was just initialized to be empty, so there
is no way bio_has_data() will return true.  So remove stale BUG_ON().
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1274,7 +1274,6 @@ static int __send_empty_flush(struct clone_info *ci)
 	ci->bio = &flush_bio;
 	ci->sector_count = 0;
 
-	BUG_ON(bio_has_data(ci->bio));
 	while ((ti = dm_table_get_target(ci->map, target_nr++)))
 		__send_duplicate_bios(ci, ti, ti->num_flush_bios, NULL);