gianfar: fix potential sk_wmem_alloc imbalance

commit db83d13 (gianfar: Fix missing sock reference when
processing TX time stamps) added a potential sk_wmem_alloc imbalance

If the new skb has a different truesize than old one, we can get a
negative sk_wmem_alloc once new skb is orphaned at TX completion.

Now we no longer early orphan skbs in dev_hard_start_xmit(), this
probably can lead to fatal bugs.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Manfred Rudigier <manfred.rudigier@omicron.at>
Cc: Claudiu Manoil <claudiu.manoil@freescale.com>
Cc: Jiajun Wu <b06378@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/gianfar.c

@@ -2063,10 +2063,9 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 			return NETDEV_TX_OK;
 		}
 
-		/* Steal sock reference for processing TX time stamps */
-		swap(skb_new->sk, skb->sk);
-		swap(skb_new->destructor, skb->destructor);
-		kfree_skb(skb);
+		if (skb->sk)
+			skb_set_owner_w(skb_new, skb->sk);
+		consume_skb(skb);
 		skb = skb_new;
 	}