Merge branch 'mptcp-rx-path-refactor'

Matthieu Baerts says:

====================
mptcp: rx path refactor

Paolo worked on this RX path refactor for these two main reasons:

- Currently, the MPTCP RX path introduces quite a bit of 'exceptional'
  accounting/locking processing WRT to plain TCP, adding up to the
  implementation complexity in a miserable way.

- The performance gap WRT plain TCP for single subflow connections is
  quite measurable.

The present refactor addresses both the above items: most of the
additional complexity is dropped, and single stream performances
increase measurably, from 55Gbps to 71Gbps in Paolo's loopback test.
As a reference, plain TCP was around 84Gbps on the same host.

The above comes to a price: the patch are invasive, even in subtle ways.

Note: patch 5/7 removes the sk_forward_alloc_get() helper, which caused
some trivial modifications in different places in the net tree: sockets,
IPv4, sched. That's why a few more people have been Cc here. Feel free
to only look at this patch 5/7.
====================

Link: https://patch.msgid.link/20250218-net-next-mptcp-rx-path-refactor-v1-0-4a47d90d7998@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>