selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

Currently trigger-hist-poll.tc uses sched_process_free to test the polling
of the histogram file. The way it does that is to run sleep, then execute
the poll.c code that polls on the sched_process_free for up to 4 seconds
to test that when sleep triggers the sched_process_free trace event, it
will update the histogram and wake the poll.c code up.

The issue is that sched_process_free trace event is called by
delayed_put_task_struct() which is called after a RCU grace period has
ended. If CONFIG_RCU_LAZY is enabled, RCU callbacks are batched together
and do not execute right away. This causes the delayed_put_task_struct()
to be called after the poll.c function finishes and it will report an
error that it did not wake up on the event. That's because the event
didn't trigger during its wait time.

Use sched_process_exit instead, which is called when a process exits and
doesn't depend on RCU callbacks that may be delayed.

Link: https://lore.kernel.org/r/20260708163436.058cc3df@gandalf.local.home
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

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