Revert "perf: protect group_leader from races that cause ctx double-free"

This reverts commit 9e4a31bc. This patch
was re-worked upstream to fix CVE-2016-6787. Refer to the upstream
discussion for details: https://lore.kernel.org/lkml/20170105231429.GA83592@beast/



The official upstream fix in this tree is commit
321027c1.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Bug: 30955111
Bug: 31095224
Bug: 148872640
Change-Id: Ib990505d0e364398d67b16e3b204b86dfdb8734d