seccomp: Add missing return in non-void function

We don't actually care about the value, since the kernel will panic
before that; but a value should nonetheless be returned, otherwise the
compiler will complain.

Fixes: 8112c4f1 ("seccomp: remove 2-phase API")
Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210111172839.640914-1-paul@crapouillou.net