Fix race on io_u->flags

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/verify.c b/verify.c
index 7d675e4..84e888b 100644
--- a/verify.c
+++ b/verify.c
@@ -444,10 +444,10 @@
 	}
 	flist_del(&io_u->list);
 	flist_add_tail(&io_u->list, &td->verify_list);
+	io_u->flags |= IO_U_F_FREE_DEF;
 	pthread_mutex_unlock(&td->io_u_lock);
 
 	pthread_cond_signal(&td->verify_cond);
-	io_u->flags |= IO_U_F_FREE_DEF;
 	return 0;
 }