CVE-2025-38588 ipv6: prevent infinite loop in rt6_nlmsg_size()

In the Linux kernel, the following vulnerability has been resolved: ipv6: prevent infinite loop in rt6nlmsgsize While testing prior patch, I was able to trigger an infinite loop in rt6nlmsgsize in the following place: listforeachentryrcusibling, &f6i-fib6siblings, fib6siblings...