tg3: Don't modify ip header fields when doing GSO

tg3 uses GSO as workaround if the hardware cannot perform TSO on certain
packets.  We should not modify the ip header fields if we do GSO on the
packet.  It happens to work by accident because GSO recalculates the IP
checksum and IP total length.

Also fix the tg3_start_xmit comment to reflect that this is the only
xmit function for all devices.
Signed-off-by: NPrashant Sreedharan <prashant@broadcom.com>
Signed-off-by: NMichael Chan <mchan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -7871,9 +7871,7 @@ static int tg3_tso_bug(struct tg3 *tp, struct sk_buff *skb)
 	return NETDEV_TX_OK;
 }
 
-/* hard_start_xmit for devices that have the 4G bug and/or 40-bit bug and
- * support TG3_FLAG_HW_TSO_1 or firmware TSO only.
- */
+/* hard_start_xmit for all devices */
 static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct tg3 *tp = netdev_priv(dev);
@@ -7927,14 +7925,14 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb) - ETH_HLEN;
 
 		if (!skb_is_gso_v6(skb)) {
+			if (unlikely((ETH_HLEN + hdr_len) > 80) &&
+			    tg3_flag(tp, TSO_BUG))
+				return tg3_tso_bug(tp, skb);
+
 			iph->check = 0;
 			iph->tot_len = htons(mss + hdr_len);
 		}
 
-		if (unlikely((ETH_HLEN + hdr_len) > 80) &&
-		    tg3_flag(tp, TSO_BUG))
-			return tg3_tso_bug(tp, skb);
-
 		base_flags |= (TXD_FLAG_CPU_PRE_DMA |
 			       TXD_FLAG_CPU_POST_DMA);