CVE-2025-71080 ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT

In the Linux kernel, the following vulnerability has been resolved: ipv6: fix a BUG in rt6getpcpuroute under PREEMPTRT On PREEMPTRT kernels, after rt6getpcpuroute returns NULL, the current task can be preempted. Another task running on the same CPU may then execute rt6makepcpuroute and successful...