UPSTREAM: cgroup: make cgroup.threads delegatable

Make cgroup.threads file delegatable.
The behavior of cgroup.threads should follow the behavior of cgroup.procs.

Signed-off-by: Roman Gushchin <guro@fb.com>
Discovered-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Change-Id: I82d23cd511122e5a75b23b26e03ccc9e43b171e5
(cherry picked from commit 4f58424d)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>