selftests: mptcp: sockopt: save nstat infos

selftests: mptcp: sockopt: save nstat infos

Similar to the way nstat information is stored in mptcp_connect.sh
and mptcp_join.sh scripts, this patch adds a similar way for
mptcp_sockopt.sh and displays the nstat information when errors
occur.

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://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-2-2ffb16a6cf35@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>