Merge branch 'mptcp-fixes'

Matthieu Baerts says:

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mptcp: fixes for v6.5

Here is a first batch of fixes for v6.5 and older.

The fixes are not linked to each others.

Patch 1 ensures subflows are unhashed before cleaning the backlog to
avoid races. This fixes another recent fix from v6.4.

Patch 2 does not rely on implicit state check in mptcp_listen() to avoid
races when receiving an MP_FASTCLOSE. A regression from v5.17.

The rest fixes issues in the selftests.

Patch 3 makes sure errors when setting up the environment are no longer
ignored. For v5.17+.

Patch 4 uses 'iptables-legacy' if available to be able to run on older
kernels. A fix for v5.13 and newer.

Patch 5 catches errors when issues are detected with packet marks. Also
for v5.13+.

Patch 6 uses the correct variable instead of an undefined one. Even if
there was no visible impact, it can help to find regressions later. An
issue visible in v5.19+.

Patch 7 makes sure errors with some sub-tests are reported to have the
selftest marked as failed as expected. Also for v5.19+.

Patch 8 adds a kernel config that is required to execute MPTCP
selftests. It is valid for v5.9+.

Patch 9 fixes issues when validating the userspace path-manager with
32-bit arch, an issue affecting v5.19+.
====================

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>