mptcp: pm: split in-kernel PM specific code

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

- pm.c had common code for all PMs

- pm_netlink.c was supposed to be about the in-kernel PM, but also had
  exported common Netlink helpers, NL events for PM userspace daemons,
  etc. quite confusing.

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

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

- 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
  - (while at it, a useless 'return;' spot by checkpatch at the end of
     mptcp_pm_nl_set_flags_all, has been removed)

The code around the PM is now less confusing, which should help for the
maintenance in the long term.

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.

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-v1-14-abef20ada03b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/Makefile

@@ -3,7 +3,7 @@ obj-$(CONFIG_MPTCP) += mptcp.o
 
 mptcp-y := protocol.o subflow.o options.o token.o crypto.o ctrl.o pm.o diag.o \
 	   mib.o pm_netlink.o sockopt.o pm_userspace.o fastopen.o sched.o \
-	   mptcp_pm_gen.o
+	   mptcp_pm_gen.o pm_kernel.o
 
 obj-$(CONFIG_SYN_COOKIES) += syncookies.o
 obj-$(CONFIG_INET_MPTCP_DIAG) += mptcp_diag.o