Use unique seed for zipf/pareto init if rand_repeat is not set

Use unique seed for zipf/pareto init if rand_repeat is not set

By default, fio uses repeatable random sequences for anything.
This makes it easy to run the same job again. But if this is
disabled, ensure that the zipf/pareto sequences are also
randomized. Re-use the normal random io offset seed for
the random distribution too.

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