Merge branch 'mptcp-add-mixed-v4-v6-support-for-the-in-kernel-pm'

Matthieu Baerts says:

====================
mptcp: add mixed v4/v6 support for the in-kernel PM

Before these patches, the in-kernel Path-Manager would not allow, for
the same MPTCP connection, having a mix of subflows in v4 and v6.

MPTCP's RFC 8684 doesn't forbid that and it is even recommended to do so
as the path in v4 and v6 are likely different. Some networks are also
v4 or v6 only, we cannot assume they all have both v4 and v6 support.

Patch 1 then removes this artificial constraint in the in-kernel PM
currently enforcing there are no mixed subflows in place, either in
address announcement or in subflow creation areas.

Patch 2 makes sure the sk_ipv6only attribute is also propagated to
subflows, just in case a new PM wouldn't respect it.

Some selftests have also been added for the in-kernel PM (patch 3).

Patches 4 to 8 are just some cleanups and small improvements in the
printed messages in the userspace PM. It is not linked to the rest but
identified when working on a related patch modifying this selftest,
already in -net:

  commit 4656d72c1efa ("selftests: mptcp: userspace: validate v4-v6 subflows mix")
---
====================

Link: https://lore.kernel.org/r/20230123-upstream-net-next-pm-v4-v6-v1-0-43fac502bfbf@tessares.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>