Adds check for rand_seed during verify phase.

Adds check for rand_seed during verify phase.

Improve data integrity checking of header (meta) data.
verify_header() will now return an additional error:
"verify: bad header rand seed ..."
The addition of the check of rand_seed helps detect stale data from
previous fio runs.
This patch also disambiguates the different data mismatches by returning
different error codes from verify_header().

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