perf/core: Detach event groups during remove_on_exec

In the Linux kernel, the following vulnerability has been resolved: perf/core: Detach event groups during remove_on_exec perf_event_remove_on_exec() removes events by calling perf_event_exit_event(). For top-level events, this removes the event from the context with DETACH_EXIT only. This can leave inconsistent group state when a removed event is a group leader and the group contains siblings without remove_on_exec. If the group was active, the surviving siblings can remain active and attached to the removed leader's sibling list, but are no longer represented by a valid group leader on the PMU context active lists. A later close of the removed leader uses DETACH_GROUP and can promote the still-active siblings from this stale group state. The next schedule-in can then add an already-linked active_list entry again, corrupting the PMU context active list. With DEBUG_LIST enabled, this is caught as a list_add double-add in merge_sched_in(). Fix this by detaching group relationships when remove_on_exec removes an event. This preserves the existing task-exit and revoke behavior, while ensuring surviving siblings are ungrouped before the removed event leaves the context.