selftests: mptcp: use KSFT_SKIP/KSFT_PASS/KSFT_FAIL

This patch uses the public var KSFT_SKIP in mptcp_lib.sh instead of
ksft_skip, and drop 'ksft_skip=4' in mptcp_join.sh.

Use KSFT_PASS and KSFT_FAIL macros instead of 0 and 1 after 'exit '
and 'ret=' in all scripts:

        exit 0 -> exit ${KSFT_PASS}
        exit 1 -> exit ${KSFT_FAIL}
         ret=0 ->  ret=${KSFT_PASS}
         ret=1 ->  ret=${KSFT_FAIL}

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-15-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>