Add 'null' verify option

For testing purposes.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 660db07..9c8ff26 100644
--- a/io_u.c
+++ b/io_u.c
@@ -666,7 +666,7 @@
 		icd->bytes_done[idx] += bytes;
 
 		if (io_u->end_io) {
-			ret = io_u->end_io(io_u);
+			ret = io_u->end_io(td, io_u);
 			if (ret && !icd->error)
 				icd->error = ret;
 		}