Don't grab stat mutex for final stat output

We should not need it, and there's a bug with getting stuck on it
that hasn't been fully debugged/understood yet. So for now, lets
just forget about the mutex for final output.

Signed-off-by: Jens Axboe <axboe@fb.com>
Allow reset of offset_increment counter

Reset offset_increment only within a sub-group of jobs, eg when
numjobs=x is used to automatically group jobs together.

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