kernel_netlink.c: clear stale routes from the IPv6 route cache

Based on reading Linux's net/ipv6/route.c, the sysctl net.ipv6.route.flush takes an integer argument n.  If n>0, the fib6 table flushes routes older than n seconds.  If n<=0, however, the sysctl assumes a maximum route age equal to net.ipv6.route.gc_interval, which defaults to 30 seconds.  This is the present behaviour of flush_routing_cache_v6().

This patch modifies the flush_routing_cache_v6() function to clear any cached routes older than 1 second.  This should increase the effective speed of route changes.