perf/core: Fix event->parent life-time issue

perf/core: Fix event->parent life-time issue

Due to an oversight in merging:

  da916e96e2de ("perf: Make perf_pmu_unregister() useable")

on top of:

  56799bc03565 ("perf: Fix hang while freeing sigtrap event")

.. it is now possible to hit put_event(EVENT_TOMBSTONE), which makes
the computer sad.

This also means that for the event->parent == EVENT_TOMBSTONE, the
put_event() matching inherit_event() has gone missing.

Previously this was done in perf_event_release_kernel() after calling
perf_remove_from_context(), but with it delegated to put_event(), this
case is now entirely missed, leading to leaks.

Fixes: da916e96e2de ("perf: Make perf_pmu_unregister() useable")
Reported-by: kernel test robot <oliver.sang@intel.com>
Tested-by: kernel test robot <oliver.sang@intel.com>
Tested-by: James Clark <james.clark@linaro.org>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Closes: https://lore.kernel.org/oe-lkp/202504131701.941039cd-lkp@intel.com
Link: https://lkml.kernel.org/r/20250415131446.GN5600@noisy.programming.kicks-ass.net