seccomp: always propagate NO_NEW_PRIVS on tsync

seccomp: always propagate NO_NEW_PRIVS on tsync

BugLink: http://bugs.launchpad.net/bugs/1553179
commit 103502a35cfce0710909da874f092cb44823ca03 upstream.

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>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>