mptcp: pm: move generic helper at the top

In prevision to another change importing all generic PM helpers from
pm_netlink.c to there.

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-v1-12-abef20ada03b@kernel.org


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