selftests: mptcp: join: add ss mptcp support check

Commands 'ss -M' are used in script mptcp_join.sh to display only MPTCP
sockets. So it must be checked if ss tool supports MPTCP in this script.

Fixes: e274f715 ("selftests: mptcp: add subflow limits test-cases")
Cc: stable@vger.kernel.org
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/20240223-upstream-net-20240223-misc-fixes-v1-7-162e87e48497@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -161,6 +161,11 @@ check_tools()
 		exit $ksft_skip
 	fi
 
+	if ! ss -h | grep -q MPTCP; then
+		echo "SKIP: ss tool does not support MPTCP"
+		exit $ksft_skip
+	fi
+
 	# Use the legacy version if available to support old kernel versions
 	if iptables-legacy -V &> /dev/null; then
 		iptables="iptables-legacy"