mptcp: fix address removal logic in mptcp_pm_nl_rm_addr

CVE-2025-68221

What is CVE-2025-68221?

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

mptcp: fix address removal logic in mptcp_pm_nl_rm_addr

Fix inverted WARN_ON_ONCE condition that prevented normal address removal counter updates. The current code only executes decrement logic when the counter is already 0 (abnormal state), while normal removals (counter > 0) are ignored.

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