selftests: mptcp: pm: get server-side flag

selftests: mptcp: pm: get server-side flag

server-side info linked to the MPTCP connect/established events can now
come from the flags, in addition to the dedicated attribute.

The attribute is now deprecated -- in favour of the new flag, and will
be removed later on.

Print this info only once.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250919-net-next-mptcp-server-side-flag-v1-4-a97a5d561a8b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>