kaweth: use skb_cow_head() to deal with cloned skbs

BugLink: https://bugs.launchpad.net/bugs/1818813

commit 39fba783 upstream.

We can use skb_cow_head() to properly deal with clones,
especially the ones coming from TCP stack that allow their head being
modified. This avoids a copy.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: James Hughes <james.hughes@raspberrypi.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/net/usb/kaweth.c

@@ -812,19 +812,13 @@ static netdev_tx_t kaweth_start_xmit(struct sk_buff *skb,
 	}
 
 	/* We now decide whether we can put our special header into the sk_buff */
-	if (skb_cloned(skb) || skb_headroom(skb) < 2) {
-		/* no such luck - we make our own */
-		struct sk_buff *copied_skb;
-		copied_skb = skb_copy_expand(skb, 2, 0, GFP_ATOMIC);
-		dev_kfree_skb_irq(skb);
-		skb = copied_skb;
-		if (!copied_skb) {
+	if (skb_cow_head(skb, 2)) {
 		kaweth->stats.tx_errors++;
 		netif_start_queue(net);
 		spin_unlock_irq(&kaweth->device_lock);
+		dev_kfree_skb_any(skb);
 		return NETDEV_TX_OK;
 	}
-	}
 
 	private_header = (__le16 *)__skb_push(skb, 2);
 	*private_header = cpu_to_le16(skb->len-2);