fio: consolidate rand_seed to uint64_t

csscope showed rand_seed was defined 4 times - each a different type.
While they are used differently, the places where they overlap
should reference the same type. This patch makes three of them the same
as suggested by Jens Axboe.

Signed-off-by: Grant Grundler <grundler@chromium.org>

Updated by me to change rand_seed option to unsigned long long
and changes related to that.

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