Merge branch 'mptcp-fix-endpoints-with-signal-and-subflow-flags'

Merge branch 'mptcp-fix-endpoints-with-signal-and-subflow-flags'

Matthieu Baerts says:

====================
mptcp: fix endpoints with 'signal' and 'subflow' flags

When looking at improving the user experience around the MPTCP endpoints
setup, I noticed that setting an endpoint with both the 'signal' and the
'subflow' flags -- as it has been done in the past by users according to
bug reports we got -- was resulting on only announcing the endpoint, but
not using it to create subflows: the 'subflow' flag was then ignored.

My initial thought was to modify IPRoute2 to warn the user when the two
flags were set, but it doesn't sound normal to ignore one of them. I
then looked at modifying the kernel not to allow having the two flags
set, but when discussing about that with Mat, we thought it was maybe
not ideal to do that, as there might be use-cases, we might break some
configs. Then I saw it was working before v5.17. So instead, I fixed the
support on the kernel side (patch 5) using Paolo's suggestion. This also
includes a fix on the options side (patch 1: for v5.11+), an explicit
deny of some options combinations (patch 2: for v5.18+), and some
refactoring (patches 3 and 4) to ease the inclusion of the patch 5.

While at it, I added a new selftest (patch 7) to validate this case --
including a modification of the chk_add_nr helper to inverse the sides
were the counters are checked (patch 6) -- and allowed ADD_ADDR echo
just after the MP_JOIN 3WHS.

The selftests modification have the same Fixes tag as the previous
commit, but no 'Cc: Stable': if the backport can work, that's good --
but it still need to be verified by running the selftests -- if not, no
need to worry, many CIs will use the selftests from the last stable
version to validate previous stable releases.
====================

Link: https://patch.msgid.link/20240731-upstream-net-20240731-mptcp-endp-subflow-signal-v1-0-c8a9b036493b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>