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: e274f7154008 ("selftests: mptcp: add subflow limits test-cases")
Cc: [e-mail]
Signed-off-by: Geliang Tang <[e-mail]>
Reviewed-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 		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"