Merge branch 'mptcp-handle-late-add_addr-selftests-skip'

Matthieu Baerts says:

====================
mptcp: handle late ADD_ADDR + selftests skip

Here are a few independent fixes related to MPTCP and its selftests:

- Patch 1: correctly handle ADD_ADDR being received after the switch to
  'fully-established'. A fix for another recent fix backported up to
  v5.14.

- Patches 2-5: properly mark some MPTCP Join subtests as 'skipped' if
  the tested kernel doesn't support the feature being validated. Some
  fixes for up to v5.13, v5.18, v6.11 and v6.18-rc1 respectively.
====================

Link: https://patch.msgid.link/20251020-net-mptcp-c-flag-late-add-addr-v1-0-8207030cb0e8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>