perf offcpu: Track child processes

When -p option used or a workload is given, it needs to handle child
processes.  The perf_event can inherit those task events
automatically.  We can add a new BPF program in task_newtask
tracepoint to track child processes.

Before:
  $ sudo perf record --off-cpu -- perf bench sched messaging
  $ sudo perf report --stat | grep -A1 offcpu
  offcpu-time stats:
            SAMPLE events:        1

After:
  $ sudo perf record -a --off-cpu -- perf bench sched messaging
  $ sudo perf report --stat | grep -A1 offcpu
  offcpu-time stats:
            SAMPLE events:      856

Signed-off-by: Namhyung Kim <[email protected]>
Cc: Blake Jones <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Milian Wolff <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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