perf bench messaging: Store chlid process pid when creating worker for process mode

perf bench messaging: Store chlid process pid when creating worker for process mode

To save pid of child processes when creating worker:
1. The messaging worker is changed to `union` type to store thread id and
   process pid.
2. Save child process pid in create_process_worker().
3. Rename `pth_tab` as `work_tab`.

Test result:

  # perf bench sched messaging
  # Running 'sched/messaging' benchmark:
  # 20 sender and receiver processes per group
  # 10 groups == 400 processes run

       Total time: 6.744 [sec]
  # perf bench sched messaging -t
  # Running 'sched/messaging' benchmark:
  # 20 sender and receiver threads per group
  # 10 groups == 400 threads run

       Total time: 5.788 [sec]

Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20230923093037.961232-4-yangjihong1@huawei.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>