perf: Support only inheriting events if cloned with CLONE_THREAD

perf: Support only inheriting events if cloned with CLONE_THREAD

Adds bit perf_event_attr::inherit_thread, to restricting inheriting
events only if the child was cloned with CLONE_THREAD.

This option supports the case where an event is supposed to be
process-wide only (including subthreads), but should not propagate
beyond the current process's shared environment.

Suggested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Marco Elver <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/lkml/YBvj6eJR%[email protected]/