backend: fix a case where we complain about no IO being done

backend: fix a case where we complain about no IO being done

If we're just doing a read phase verify, then we clear
bytes_done before we can check it at the end. Ensure that we
always notice if we did IO, so we don't spew this warning:

job_0: No I/O performed by libaio, perhaps try --debug=io option for details?

when we actually did do exactly what we had to.

Signed-off-by: Jens Axboe <axboe@kernel.dk>