dst4_mtu
room = dst4_mtu(&rt->dst);
dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu);
(skb->len <= dst4_mtu(&rt->dst) &&
val = dst4_mtu(dst);
if (skb->len+encap > dst4_mtu(&rt->dst) && (ntohs(iph->frag_off) & IP_DF)) {
if (nskb->len > dst4_mtu(skb_dst(nskb)))
tcp_sync_mss(newsk, dst4_mtu(dst));
dmtu = dst4_mtu(dst);
mtu = dst4_mtu(&rt->dst) - t_hlen;