init: refactor random seed setting

init: refactor random seed setting

td->rand_seed was modified in three different places. Put all this code
in setup_random_seeds() to make it easier to understand and more
maintanable. Also put setup_random_seeds() next to the other
random-seed-related functions in init.c.

init_rand_seed() was called in three different places for fio's main random
number generators. Also put these three sets of invocations in the same
place.

Always initialize all of fio's main set of random states instead of
skipping some for sequential workloads. This makes debugging easier.

No functional change.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>