seccomp: split mode setting routines

Separates the two mode setting paths to make things more readable with
fewer #ifdefs within function bodies.

Change-Id: I7724e5757420f2519675fdf365c9fb3692b9cab4
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>