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>

tools/testing/selftests/net/mptcp/mptcp_sockopt.sh

@@ -169,6 +169,11 @@ do_transfer()
 		cmsg+=",TCPINQ"
 	fi
 
+	NSTAT_HISTORY=/tmp/${listener_ns}.nstat ip netns exec ${listener_ns} \
+		nstat -n
+	NSTAT_HISTORY=/tmp/${connector_ns}.nstat ip netns exec ${connector_ns} \
+		nstat -n
+
 	timeout ${timeout_test} \
 		ip netns exec ${listener_ns} \
 			$mptcp_connect -t ${timeout_poll} -l -M 1 -p $port -s ${srv_proto} -c "${cmsg}" \
@@ -189,14 +194,20 @@ do_transfer()
 	wait $spid
 	local rets=$?
 
+	NSTAT_HISTORY=/tmp/${listener_ns}.nstat ip netns exec ${listener_ns} \
+		nstat | grep Tcp > /tmp/${listener_ns}.out
+	NSTAT_HISTORY=/tmp/${connector_ns}.nstat ip netns exec ${connector_ns} \
+		nstat | grep Tcp > /tmp/${connector_ns}.out
+
 	print_title "Transfer ${ip:2}"
 	if [ ${rets} -ne 0 ] || [ ${retc} -ne 0 ]; then
 		mptcp_lib_pr_fail "client exit code $retc, server $rets"
 		echo -e "\nnetns ${listener_ns} socket stat for ${port}:" 1>&2
 		ip netns exec ${listener_ns} ss -Menita 1>&2 -o "sport = :$port"
-
+		cat /tmp/${listener_ns}.out
 		echo -e "\nnetns ${connector_ns} socket stat for ${port}:" 1>&2
 		ip netns exec ${connector_ns} ss -Menita 1>&2 -o "dport = :$port"
+		cat /tmp/${connector_ns}.out
 
 		mptcp_lib_result_fail "transfer ${ip}"