seccomp: extract check/assign mode helpers

To support splitting mode 1 from mode 2, extract the mode checking and
assignment logic into common functions.

Change-Id: Ie0160b0d9dc60a88e47e4926a2aae83a99b3b2db
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Git-commit: b8a9cff6dbe9cfddbb4d17e2dea496e523544687
Git-repo: https://www.codeaurora.org/cgit/external/gigabyte/kernel/msm


Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>