CVE-2026-63969 ipv6: fix possible infinite loop in rt6_fill_node()

In the Linux kernel, the following vulnerability has been resolved: ipv6: fix possible infinite loop in rt6fillnode Sashiko reported this issue 1. Apply the same fix as commit f8d8ce1b515a "ipv6: fix possible infinite loop in fib6infousesdev". Writers holding tb6lock can listdelrcu&rt-fib6sibling...