Fix start delay being the same across threads

Fix start delay being the same across threads

Two issues here:

1) We copy the start_delay into the original thread, which ends up
   offsetting job 2..N off the thread 1 start delay
2) We don't initialize the start delay random seed prior to
   generating the offsets

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