perf: Fix check before add_event_to_groups() in perf_group_detach()

Events should only be added to a groups rb tree if they have not been
removed from their context by list_del_event(). Since remove_on_exec
made it possible to call list_del_event() on individual events before
they are detached from their group, perf_group_detach() should check each
sibling's attach_state before calling add_event_to_groups() on it.

Fixes: 2e498d0a74e5 ("perf: Add support for event removal on exec")
Signed-off-by: Budimir Markovic <markovicbudimir@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/ZBFzvQV9tEqoHEtH@gentoo
