selftests: mptcp: join: add ss mptcp support check

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