selftests/seccomp: Refactor get_proc_stat to split out file reading code

This splits up the get_proc_stat function to make it so we can use it as a
generic helper to read the nth field from multiple different files, versus
replicating the logic in multiple places.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503080958.20220-3-sargun@sargun.me