perf: don't leave group_entry on sibling list (use-after-free)

When perf_group_detach is called on a group leader,
it should empty its sibling list. Otherwise, when
a sibling is later deallocated, list_del_event()
removes the sibling's group_entry from its current
list, which can be the now-deallocated group leader's
sibling list (use-after-free bug).

Bug: 32402548
Change-Id: I99f6bc97c8518df1cb0035814368012ba72ab1f1
Signed-off-by: John Dias <joaodias@google.com>
Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: 6b6cfb2362f09553b46b3b7e5684b16b6e53e373
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>