selftests: kselftest_harness: use exit code to store skip

We always use skip in combination with exit_code being 0
(KSFT_PASS). This are basic KSFT / KTAP semantics.
Store the right KSFT_* code in exit_code directly.

This makes it easier to support tests reporting other
extended KSFT_* codes like XFAIL / XPASS.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>