hyperv: Fix a bug in netvsc_start_xmit()

After the packet is successfully sent, we should not touch the skb
as it may have been freed. This patch is based on the work done by
Long Li <longli@microsoft.com>.

In this version of the patch I have fixed issues pointed out by David.
David, please queue this up for stable.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Long Li <longli@microsoft.com>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -387,6 +387,7 @@ static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net)
 	int  hdr_offset;
 	u32 net_trans_info;
 	u32 hash;
+	u32 skb_length = skb->len;
 
 
 	/* We will atmost need two pages to describe the rndis
@@ -562,7 +563,7 @@ static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net)
 
 drop:
 	if (ret == 0) {
-		net->stats.tx_bytes += skb->len;
+		net->stats.tx_bytes += skb_length;
 		net->stats.tx_packets++;
 	} else {
 		kfree(packet);