Merge branch 'mptcp-pm-code-reorganisation'

Matthieu Baerts says:

====================
mptcp: pm: code reorganisation

Before this series, the PM code was dispersed in different places:

- pm.c had common code for all PMs.

- pm_netlink.c was initially only about the in-kernel PM, but ended up
  also getting exported common helpers, callbacks used by the different
  PMs, NL events for PM userspace daemon, etc. quite confusing.

- pm_userspace.c had userspace PM only code, but it was using "specific"
  in-kernel PM helpers according to their names.

To clarify the code, a reorganisation is suggested here, only by moving
code around, and small helper renaming to avoid confusions:

- pm_netlink.c now only contains common PM generic Netlink code:
  - PM events: this code was already there
  - shared helpers around Netlink code that were already there as well
  - shared Netlink commands code from pm.c

- pm_kernel.c now contains only code that is specific to the in-kernel
  PM. Now all functions are either called from:
  - pm.c: events coming from the core, when this PM is being used
  - pm_netlink.c: for shared Netlink commands
  - mptcp_pm_gen.c: for Netlink commands specific to the in-kernel PM
  - sockopt.c: for the exported counters per netns

- pm.c got many code from pm_netlink.c:
  - helpers used from both PMs and not linked to Netlink
  - callbacks used by different PMs, e.g. ADD_ADDR management
  - some helpers have been renamed to remove the '_nl' prefix, and some
    have been marked as 'static'.

- protocol.h has been updated accordingly:
  - some helpers no longer need to be exported
  - new ones needed to be exported: they have been prefixed if needed.

The code around the PM is now less confusing, which should help for the
maintenance in the long term, and the introduction of a PM Ops.

This will certainly impact future backports, but because other cleanups
have already done recently, and more are coming to ease the addition of
a new path-manager controlled with BPF (struct_ops), doing that now
seems to be a good time. Also, many issues around the PM have been fixed
a few months ago while increasing the code coverage in the selftests, so
such big reorganisation can be done with more confidence now.

Note that checkpatch, when used with --max-line-length=80, will complain
 about lines being over the 80 limits, but these warnings were already
there before moving the code around.

Also, patch 1 is not directly related to the code reorganisation, but it
was a remaining cleanup that we didn't upstream before, because it was
conflicting with another patch that has been sent for inclusion to the
net tree.
====================

Link: https://patch.msgid.link/20250307-net-next-mptcp-pm-reorg-v1-0-abef20ada03b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>