selftests: mptcp: add -m with ss in case of errors

Recently, we had an issue where getting info about the memory would have
helped better understanding what went wrong.

Let add it just in case for later.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-4-2ffb16a6cf35@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>