RTAX_ADVMSS
metric == RTAX_ADVMSS ||
u32 advmss = dst_metric_raw(dst, RTAX_ADVMSS);
#define fib_advmss fib_metrics->metrics[RTAX_ADVMSS-1]
len = put_rtax(mx, len, RTAX_ADVMSS, rt->rt_mtu - 40);
if (type == RTAX_ADVMSS && val > 65535 - 40)