CVE-2026-98438: mptcp: options: fix uninit-value in mptcp_write_data_fin

In the Linux kernel, the following vulnerability has been resolved:

mptcp: options: fix uninit-value in mptcpwritedatafin

When sending a DATAFIN without data, and because the DATAFIN occupies 1 octet of the connection-level sequence space [1], it is then required to add a DSS mapping with specific values.

If the checksum has been negotiated, it also needs to be computed, and included in the outgoing packet, and thus the initial csum data needs to be reset to 0 as well. This is no longer the case since commit cfcceb7a39fc ("tcp: shrink per-packet memset in tcptransmitskb()"), because the whole extcopy structure is no longer zeroed by default.

This seems to be the only case where usemap is changed and set afterwards, so initialising the csum field only in this case, along with other fields for this specific case.