Wrap thread_data in thread_segment

Wrap thread_data in thread_segment

No functional changes in this patch, just in preparation for having
multiple shm segments for the thread_data structures. They are getting
very large, and since it's hard to know upfront how many we need, let's
instead refactor the code a bit to allow us to add chunks of them as
needed when parsing the job file (or command line).

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