Merge branch 'mptcp-pm-listener-events-selftests-cleanup'

Matthieu Baerts says:

====================
mptcp: PM listener events + selftests cleanup

Thanks to the patch 6/11, the MPTCP path manager now sends Netlink events
when MPTCP listening sockets are created and closed. The reason why it is
needed is explained in the linked ticket [1]:

  MPTCP for Linux, when not using the in-kernel PM, depends on the
  userspace PM to create extra listening sockets before announcing
  addresses and ports. Let's call these "PM listeners".

  With the existing MPTCP netlink events, a userspace PM can create
  PM listeners at startup time, or in response to an incoming connection.
  Creating sockets in response to connections is not optimal: ADD_ADDRs
  can't be sent until the sockets are created and listen()ed, and if all
  connections are closed then it may not be clear to the userspace
  PM daemon that PM listener sockets should be cleaned up.

  Hence this feature request: to add MPTCP netlink events for listening
  socket close & create, so PM listening sockets can be managed based
  on application activity.

  [1] https://github.com/multipath-tcp/mptcp_net-next/issues/313

Selftests for these new Netlink events have been added in patches 9,11/11.

The remaining patches introduce different cleanups and small improvements
in MPTCP selftests to ease the maintenance and the addition of new tests.
====================

Link: https://lore.kernel.org/r/20221130140637.409926-1-matthieu.baerts@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>