seccomp: always propagate NO_NEW_PRIVS on tsync

Before this patch, a process with some permissive seccomp filter
that was applied by root without NO_NEW_PRIVS was able to add
more filters to itself without setting NO_NEW_PRIVS by setting
the new filter from a throwaway thread with NO_NEW_PRIVS.

Signed-off-by: Jann Horn <jann@thejh.net>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Bug: 36656103
(cherry-picked from commit 103502a3)
Signed-off-by: Paul Lawrence <paullawrence@google.com>

Change-Id: I5abd7daab9172f1dfd53e11706b7c7f331f2f4f1