mptcp: allow subflow rcv wnd to shrink

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SUSE CVE-2026-53183

In the Linux kernel, the following vulnerability has been resolved: mptcp: allow subflow rcv wnd to shrink In MPTCP connection, the window field in the TCP header refers to the MPTCP-level rcvnxt and it's right edge should not move backward. Such constraint is enforced at DSS option generation...