selftests: mptcp: userspace pm: validate deny-join-id0 flag

The previous commit adds the MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 flag. Make
sure it is correctly announced by the other peer when it has been
received.

pm_nl_ctl will now display 'deny_join_id0:1' when monitoring the events,
and when this flag was set by the other peer.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 702c2f64 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-3-40171884ade8@kernel.org


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