Merge branch 'mptcp-support-msg_eor-and-small-cleanups'

Matthieu Baerts says:

====================
mptcp: support MSG_EOR and small cleanups

This series contains various unrelated patches:

- Patches 1 & 2: support MSG_EOR instead of ignoring it.

- Patch 3: avoid duplicated code in TCP and MPTCP by using a new helper.

- Patch 4: adapt test to reproduce bug and increase code coverage.
====================

Link: https://patch.msgid.link/20260403-net-next-mptcp-msg_eor-misc-v1-0-b0b33bea3fed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>