backend: count iterative bytes for progress

Commit ac28d9053679 changed it so we don't clear ->bytes_done[]
necessarily for each loop. However, this introduced a busy loop
where we this we did IO and continue to loop, but we really
didn't.

Fixes: ac28d9053679 ("Fixup bw/iops logging for short runs")
Signed-off-by: Jens Axboe <axboe@fb.com>