CVE-2018-3639 (x86)

Use PR_SPEC_FORCE_DISABLE in seccomp() because seccomp does not allow to
widen restrictions.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>