perf/core: Fix event timekeeping merge

perf/core: Fix event timekeeping merge

Due to an oversight in merging:

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

on top of:

  a3c3c66670ce ("perf/core: Fix child_total_time_enabled accounting bug at task exit")

the timekeeping fix from this latter patch got undone.

Redo it.

Fixes: da916e96e2de ("perf: Make perf_pmu_unregister() useable")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20250417080815.GI38216@noisy.programming.kicks-ass.net