mptcp: fix address removal logic in mptcp_pm_nl_rm_addr

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.