CVE-2026-53214 ipv6: Fix a potential NPD in cleanup_prefix_route()

In the Linux kernel, the following vulnerability has been resolved: ipv6: Fix a potential NPD in cleanupprefixroute addrconfgetprefixroute can return the fib6nullentry sentinel entry which has a NULL fib6table pointer. Therefore, before setting the route's expiration time, check that we are not...