selftests: kselftest_harness: use KSFT_* exit codes

Now that we no longer need low exit codes to communicate
assertion steps - use normal KSFT exit codes.

Acked-by: Kees Cook <keescook@chromium.org>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>