FROMLIST: proc: Add LSM hook checks to /proc/<tid>/timerslack_ns

As requested, this patch checks the existing LSM hooks
task_getscheduler/task_setscheduler when reading or modifying
the task's timerslack value.

Previous versions added new get/settimerslack LSM hooks, but
since they checked the same PROCESS__SET/GETSCHED values as
existing hooks, it was suggested we just use the existing ones.

Cc: Kees Cook <keescook@chromium.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
CC: Arjan van de Ven <arjan@linux.intel.com>
Cc: Oren Laadan <orenl@cellrox.com>
Cc: Ruchi Kandoi <kandoiruchi@google.com>
Cc: Rom Lemarchand <romlem@android.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Colin Cross <ccross@android.com>
Cc: Nick Kralevich <nnk@google.com>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Elliott Hughes <enh@google.com>
Cc: James Morris <jmorris@namei.org>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: linux-security-module@vger.kernel.org
Cc: selinux@tycho.nsa.gov
Signed-off-by: John Stultz <john.stultz@linaro.org>

FROMLIST URL: https://lkml.org/lkml/2016/7/21/523


Change-Id: Id157d10e2fe0b85f1be45035a6117358a42af028
(Cherry picked back to common/android-4.4)
Signed-off-by: John Stultz <john.stultz@linaro.org>