Remove use of OS provided random functions

We added the internal random generator a long time ago, and kept
the OS variant around as an opt-in feature with using use_os_rand=1.
We defaulted to using the fio provided one, and I doubt that
anyone has used the option.

The time has come to kill it.

Signed-off-by: Jens Axboe <axboe@fb.com>
Get rid of __ prefix for internal frand state

It made (some) sense when we had both OS and frand in anon unions,
but now lets just kill it.

Signed-off-by: Jens Axboe <axboe@fb.com>