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 mptcp_write_data_fin

When sending a DATA_FIN without data, and because the DATA_FIN 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 __tcp_transmit_skb()"),
because the whole ext_copy structure is no longer zeroed by default.

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