Adds verify_only option

When this option is set, a dry run (no actual io is performed) of the
workload will be done in order to compute the numberio for each block
header without overwriting the data on disk. Then, do_verify() will be
effectively verifying data that was written in a previous fio run.
In the case that "loops" is set to more than 1, do_verify() will delay
the verification of numberio to the last iteration when the same
numberio state that would have been written to disk in a previous
fio run has been reached.

Signed-off-by: Juan Casse <jcasse@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>