Reenable io_u->buf_filled_len optimization

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/io_u.c b/io_u.c
index 428b312..54f8685 100644
--- a/io_u.c
+++ b/io_u.c
@@ -1147,6 +1147,7 @@
 
 	p = io_u->xfer_buf;
 	boffset = io_u->offset;
+	io_u->buf_filled_len = 0;
 
 	for (i = 0; i < nr_blocks; i++) {
 		/*