Merge branch 'mptcp-msg_fastopen-and-tfo-listener-side-support'

Matthieu Baerts says:

====================
mptcp: MSG_FASTOPEN and TFO listener side support

Before this series, only the initiator of a connection was able to combine
both TCP FastOpen and MPTCP when using TCP_FASTOPEN_CONNECT socket option.

These new patches here add (in theory) the full support of TFO with MPTCP,
which means:

 - MSG_FASTOPEN sendmsg flag support (patch 1/8)
 - TFO support for the listener side (patches 2-5/8)
 - TCP_FASTOPEN socket option (patch 6/8)
 - TCP_FASTOPEN_KEY socket option (patch 7/8)

To support TFO for the server side, a few preparation patches are needed
(patches 2 to 5/8). Some of them were inspired by a previous work from
Benjamin Hesmans.

Note that TFO support with MPTCP has been validated with selftests
(patch 8/8) but also with Packetdrill tests running with a modified
but still very WIP version supporting MPTCP. Both the modified tool
and the tests are available online:

  https://github.com/multipath-tcp/packetdrill/
====================

Link: https://lore.kernel.org/r/20221125222958.958636-1-matthieu.baerts@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>