tg3: fix tigon3_dma_hwbug_workaround()

Ari got kernel panics using tg3 NIC, and bisected to 2669069a "tg3:
enable transmit time stamping."

This is because tigon3_dma_hwbug_workaround() might alloc a new skb and
free the original. We panic when skb_tx_timestamp() is called on freed
skb.
Reported-by: NAri Savolainen <ari.m.savolainen@gmail.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tg3.c

@@ -6029,12 +6029,12 @@ static void tg3_tx_skb_unmap(struct tg3_napi *tnapi, u32 entry, int last)
 
 /* Workaround 4GB and 40-bit hardware DMA bugs. */
 static int tigon3_dma_hwbug_workaround(struct tg3_napi *tnapi,
-				       struct sk_buff *skb,
+				       struct sk_buff **pskb,
 				       u32 *entry, u32 *budget,
 				       u32 base_flags, u32 mss, u32 vlan)
 {
 	struct tg3 *tp = tnapi->tp;
-	struct sk_buff *new_skb;
+	struct sk_buff *new_skb, *skb = *pskb;
 	dma_addr_t new_addr = 0;
 	int ret = 0;
 
@@ -6076,7 +6076,7 @@ static int tigon3_dma_hwbug_workaround(struct tg3_napi *tnapi,
 	}
 
 	dev_kfree_skb(skb);
-
+	*pskb = new_skb;
 	return ret;
 }
 
@@ -6305,7 +6305,7 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		 */
 		entry = tnapi->tx_prod;
 		budget = tg3_tx_avail(tnapi);
-		if (tigon3_dma_hwbug_workaround(tnapi, skb, &entry, &budget,
+		if (tigon3_dma_hwbug_workaround(tnapi, &skb, &entry, &budget,
 						base_flags, mss, vlan))
 			goto out_unlock;
 	}