mptcp: pm: make mptcp_pm_add_addr_send_ack static

mptcp: pm: make mptcp_pm_add_addr_send_ack static

Only used in pm.c.

Note that the signature is added above: it is easier than moving the
code around, because this helper depends on mptcp_pm_schedule_work which
is declared below.

While at it, explicitly mark it as to be called while pm->lock is held.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260605-net-next-mptcp-add-addr6-port-ts-v2-13-758e7ca73f4d@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>