Merge branch 'mptcp-fallback-to-tcp-after-3-mpc-drop-cache'

Merge branch 'mptcp-fallback-to-tcp-after-3-mpc-drop-cache'

Matthieu Baerts says:

====================
mptcp: fallback to TCP after 3 MPC drop + cache

The SYN + MPTCP_CAPABLE packets could be explicitly dropped by firewalls
somewhere in the network, e.g. if they decide to drop packets based on
the TCP options, instead of stripping them off.

The idea of this series is to fallback to TCP after 3 SYN+MPC drop
(patch 2). If the connection succeeds after the fallback, it very likely
means a blackhole has been detected. In this case (patch 3), MPTCP can
be disabled for a certain period of time, 1h by default. If after this
period, MPTCP is still blocked, the period is doubled. This technique is
inspired by the one used by TCP FastOpen.

This should help applications which want to use MPTCP by default on the
client side if available.
====================

Link: https://patch.msgid.link/20240909-net-next-mptcp-fallback-x-mpc-v1-0-da7ebb4cd2a3@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>