ipv4: tcp: clean up tcp_v4_early_demux()

Use same header helpers than tcp_v6_early_demux() because they
are a bit faster, and as they make IPv4/IPv6 versions look
the same.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -1918,7 +1918,6 @@ EXPORT_SYMBOL(tcp_v4_do_rcv);
 
 void tcp_v4_early_demux(struct sk_buff *skb)
 {
-	struct net *net = dev_net(skb->dev);
 	const struct iphdr *iph;
 	const struct tcphdr *th;
 	struct sock *sk;
@@ -1926,16 +1925,16 @@ void tcp_v4_early_demux(struct sk_buff *skb)
 	if (skb->pkt_type != PACKET_HOST)
 		return;
 
-	if (!pskb_may_pull(skb, ip_hdrlen(skb) + sizeof(struct tcphdr)))
+	if (!pskb_may_pull(skb, skb_transport_offset(skb) + sizeof(struct tcphdr)))
 		return;
 
 	iph = ip_hdr(skb);
-	th = (struct tcphdr *) ((char *)iph + ip_hdrlen(skb));
+	th = tcp_hdr(skb);
 
 	if (th->doff < sizeof(struct tcphdr) / 4)
 		return;
 
-	sk = __inet_lookup_established(net, &tcp_hashinfo,
+	sk = __inet_lookup_established(dev_net(skb->dev), &tcp_hashinfo,
 				       iph->saddr, th->source,
 				       iph->daddr, ntohs(th->dest),
 				       skb->skb_iif);