io_u: sequence random buflen generation individually

io_u: sequence random buflen generation individually

For trimwrite, we rely on write+trim generating identical
buffer lengths, if they have identical settings. If we use
bssplit, then we generate them using a random generator
according to the percentages set. This means we may not get
the same offsets in sequence.

Split the seeding into separate random generators, and ensure
that we seed the write and trim ones identically, if we are
doing trim writes.

Fixes: https://github.com/axboe/fio/issues/399
Signed-off-by: Jens Axboe <axboe@kernel.dk>