io_u: calculate incremental residuals correctly

We need to use the transfer buffer length for the residual calculation,
otherwise multiple shorts on the same IO will not calculate the right
value.

Fixes: 5fff954 ("Add support for >= 4G block sizes")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_u.c

@@ -1977,7 +1977,7 @@ static void io_completed(struct thread_data *td, struct io_u **io_u_ptr,
 	td->last_ddir = ddir;
 
 	if (!io_u->error && ddir_rw(ddir)) {
-		unsigned long long bytes = io_u->buflen - io_u->resid;
+		unsigned long long bytes = io_u->xfer_buflen - io_u->resid;
 		int ret;
 
 		/*