Merge branch 'mptcp-add-tcp_maxseg-sockopt-support'

Matthieu Baerts says:

====================
mptcp: add TCP_MAXSEG sockopt support

The TCP_MAXSEG socket option was not supported by MPTCP, mainly because
it has never been requested before. But there are still valid use-cases,
e.g. with HAProxy.

- Patch 1 is a small cleanup patch in the MPTCP sockopt file.

- Patch 2 expose some code from TCP, to avoid duplicating it in MPTCP.

- Patch 3 adds TCP_MAXSEG sockopt support in MPTCP.

- Patch 4 is not related to the others, it fixes a typo in a comment.

Note that the new TCP_MAXSEG sockopt support has been validated by a new
packetdrill script on the MPTCP CI:

  https://github.com/multipath-tcp/packetdrill/pull/161

v1: https://lore.kernel.org/20250716-net-next-mptcp-tcp_maxseg-v1-0-548d3a5666f6@kernel.org
====================

Link: https://patch.msgid.link/20250719-net-next-mptcp-tcp_maxseg-v2-0-8c910fbc5307@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>