[IPV6]: Fix ipv6 GSO payload length

Fix ipv6 GSO payload length calculation.

The ipv6 payload length excludes the ipv6 base header length and so
must be subtracted.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -119,7 +119,8 @@ unlock:
 
 	for (skb = segs; skb; skb = skb->next) {
 		ipv6h = skb->nh.ipv6h;
-		ipv6h->payload_len = htons(skb->len - skb->mac_len);
+		ipv6h->payload_len = htons(skb->len - skb->mac_len -
+					   sizeof(*ipv6h));
 	}
 
 out: