Linux IPv6 improvement: Routing cache on demand

This post details the implementation and upstream contribution of an on-demand routing cache creation mechanism for Linux IPv6. This solution addresses scalability issues in large IPv6 deployments by significantly reducing the size of the IPv6 routing tree. Key technical contributions include sharing gateway routes for peers with default PMTU, implementing per-CPU entries for gateway routes to mitigate cache bouncing, and ensuring compatibility with subsystems that expect '/128' routes. The post also highlights bug fixes related to '/128' routing entries and optimizations for `fib6_lookup()`.