Merge branch 'mptcp-close-subflow-when-receiving-tcp-fin-and-misc'

Matthieu Baerts says:

====================
mptcp: close subflow when receiving TCP+FIN and misc.

Here are different fixes:

Patch 1 closes the subflow after having received a FIN, instead
of leaving it half-closed until the end of the MPTCP connection.
A fix for v5.12.

Patch 2 validates the previous patch.

Patch 3 is a fix for a recent fix to check both directions for the
backup flag. It can follow the 'Fixes' commit and be backported up
to v5.7.

Patch 4 adds a missing \n at the end of pr_debug(), causing debug
messages to be displayed with a delay, which confuses the debugger.
A fix for v5.6.
====================

Link: https://patch.msgid.link/20240826-net-mptcp-close-extra-sf-fin-v1-0-905199fe1172@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>