tg3: fix ipv6 header length computation

tg3_start_xmit() makes the wrong assumption for TSOV6 that skb->head
doesnt include any payload data.

if (skb_is_gso_v6(skb))
	hdr_len = skb_headlen(skb) - ETH_HLEN;

This is not true anymore after commit f07d960df3 (tcp: avoid frag
allocation for small frames)

We should instead use : skb_transport_offset(skb) + tcp_hdrlen(skb)

Its also true for IPv4

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Matt Carlson <mcarlson@broadcom.com>
CC: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -6667,14 +6667,9 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		iph = ip_hdr(skb);
 		tcp_opt_len = tcp_optlen(skb);
 
-		if (skb_is_gso_v6(skb)) {
-			hdr_len = skb_headlen(skb) - ETH_HLEN;
-		} else {
-			u32 ip_tcp_len;
-
-			ip_tcp_len = ip_hdrlen(skb) + sizeof(struct tcphdr);
-			hdr_len = ip_tcp_len + tcp_opt_len;
+		hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb) - ETH_HLEN;
 
+		if (!skb_is_gso_v6(skb)) {
 			iph->check = 0;
 			iph->tot_len = htons(mss + hdr_len);
 		}