Refactor the create_worker() helper:
1. Modify the return value and use pthread pointer as a parameter to
   facilitate value assignment in create_worker().
2. The thread worker creation and process worker creation are abstracted
   into independent helpers.

No functional change.

Test result:

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

       Total time: 6.332 [sec]
  # perf bench sched messaging -t
  # Running 'sched/messaging' benchmark:
  # 20 sender and receiver threads per group
  # 10 groups == 400 threads run

       Total time: 5.545 [sec]
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20230923093037.961232-3-yangjihong1@huawei.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>