CVE-2026-97568

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

mptcp: syncookies: remember the request backup flag

Instead of using an uninitialised bit when copying the info in subflow_ulp_clone().

To fix this, no need to extend the join_entry structure: backup is coming from struct mptcp_subflow_request_sock, only one bit. Do the same here by using one bit for both.