seccomp: create internal mode-setting function

In preparation for having other callers of the seccomp mode setting
logic, split the prctl entry point away from the core logic that performs
seccomp mode setting.

Change-Id: Ifbbba54931806d62e2d7fa65d0eb303a147ea021
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Git-commit: 2a30a4386e4a7e1283157c4cf4cfcc0306b22ac8
Git-repo: https://www.codeaurora.org/cgit/external/gigabyte/kernel/msm


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