Merge branch 'mptcp-Exchange-MPTCP-DATA_FIN-DATA_ACK-before-TCP-FIN'

Mat Martineau says:

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mptcp: Exchange MPTCP DATA_FIN/DATA_ACK before TCP FIN

This series allows the MPTCP-level connection to be closed with the
peers exchanging DATA_FIN and DATA_ACK according to the state machine in
appendix D of RFC 8684. The process is very similar to the TCP
disconnect state machine.

The prior code sends DATA_FIN only when TCP FIN packets are sent, and
does not allow for the MPTCP-level connection to be half-closed.

Patch 8 ("mptcp: Use full MPTCP-level disconnect state machine") is the
core of the series. Earlier patches in the series have some small fixes
and helpers in preparation, and the final four small patches do some
cleanup.
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Signed-off-by: David S. Miller <davem@davemloft.net>