tg3: fix ipv6 header length computation

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 6fd84d5d45 (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>