perf bench messaging: Kill child processes when exit abnormally in process mode

When exit abnormally in process mode, customize SIGINT and SIGTERM signal
handler to kill the forked child processes.

Before:

  # perf bench sched messaging -l 1000000 -g 1 &
  [1] 8519
  # # Running 'sched/messaging' benchmark:

  # pgrep sched-messaging | wc -l
  41
  # kill -15 8519
  [1]+  Terminated              perf bench sched messaging -l 1000000 -g 1
  # pgrep sched-messaging | wc -l
  40

After:

  # perf bench sched messaging -l 1000000 -g 1 &
  [1] 8472
  # # Running 'sched/messaging' benchmark:

  # pgrep sched-messaging | wc -l
  41
  # kill -15 8472
  [1]+  Exit 1                  perf bench sched messaging -l 1000000 -g 1
  # pgrep sched-messaging | wc -l
  0

Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20230923093037.961232-5-yangjihong1@huawei.com


[ namhyung: fix a whitespace ]
Signed-off-by: Namhyung Kim <namhyung@kernel.org>