mptcp: pm: export mptcp_remote_address

In a following commit, the 'remote_address' helper will need to be used
from different files.

It is then exported, and prefixed with 'mptcp_', similar to
'mptcp_local_address'.

No behavioural changes intended.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250307-net-next-mptcp-pm-reorg-v1-11-abef20ada03b@kernel.org


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