backend: only check bytes_done for engines without FIO_NOIO

This basically reverts the functional part of commit 2557f7e6.
Instead of having to fiddle with the io_u manually, have
the backend check whether we expect data to have been transerred
or not.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/backend.c b/backend.c
index 3b42c4d..1c944d6 100644
--- a/backend.c
+++ b/backend.c
@@ -726,7 +726,7 @@
 
 		if (ret < 0)
 			break;
-		if (!ddir_rw_sum(bytes_done))
+		if (!ddir_rw_sum(bytes_done) && !(td->io_ops->flags & FIO_NOIO))
 			continue;
 
 		if (!in_ramp_time(td) && should_check_rate(td, bytes_done)) {