rt6_nlmsg_size
static size_t rt6_nlmsg_size(struct fib6_info *f6i);
skb = nlmsg_new(rt6_nlmsg_size(rt), gfp_any());
sz = rt6_nlmsg_size(rt);
sz = max(rt6_nlmsg_size(rt), sz << 1);
skb = nlmsg_new(rt6_nlmsg_size(rt), gfp_any());
skb = nlmsg_new(rt6_nlmsg_size(f6i), GFP_KERNEL);