mptcp: options: reset DSS fields in case of unexpected size

mptcp: options: reset DSS fields in case of unexpected size

A remote peer could send a malformed DSS with a wrong size, followed by
another DSS or MPC + Data. In this case, the first suboption will be
ignored, but leaving some fields written, which could lead to
inconsistency or access uninitialized data.

Explicitly reset the fields that could have been modified in case of
unexpected size.

Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260728-net-mptcp-misc-fixes-7-2-rc6-v1-0-f7e2d229159d%40kernel.org?part=1
Fixes: 648ef4b88673 ("mptcp: Implement MPTCP receive path")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-1-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>