CVE-2026-63968 ipv6: fix possible infinite loop in fib6_select_path()

In the Linux kernel, the following vulnerability has been resolved: ipv6: fix possible infinite loop in fib6selectpath Found while auditing the same pattern Sashiko reported in rt6fillnode 1. Apply the same fix as commit f8d8ce1b515a "ipv6: fix possible infinite loop in fib6infousesdev". Writers...