Merge branch 'mptcp-pm-in-kernel-fullmesh-endp-nb-bind-cases'

Matthieu Baerts says:

====================
mptcp: pm: in-kernel: fullmesh endp nb + bind cases

Here is a small optimisation for the in-kernel PM, joined by a small
behavioural change to avoid confusions, and followed by a few more
tests.

- Patch 1: record fullmesh endpoints numbers, not to iterate over all
  endpoints to check if one is marked as fullmesh.

- Patch 2: when at least one endpoint is marked as fullmesh, only use
  these endpoints when reacting to an ADD_ADDR, even if there are no
  endpoints for this IP family: this is less confusing.

- Patch 3: reduce duplicated code to prepare the next patch.

- Patch 4: extra "bind" cases: the listen socket restrict the bind to
  one IP address, not allowing MP_JOIN to extra IP addresses, except if
  another listening socket accepts them.
====================

Link: https://patch.msgid.link/20251101-net-next-mptcp-fm-endp-nb-bind-v1-0-b4166772d6bb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>