lan78xx: use skb_cow_head() to deal with cloned skbs

commit d4ca7359 upstream.

We need to ensure there is enough headroom to push extra header,
but we also need to check if we are allowed to change headers.

skb_cow_head() is the proper helper to deal with this.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: James Hughes <james.hughes@raspberrypi.org>
Cc: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/usb/lan78xx.c

@@ -2050,14 +2050,9 @@ static struct sk_buff *lan78xx_tx_prep(struct lan78xx_net *dev,
 {
 	u32 tx_cmd_a, tx_cmd_b;
 
-	if (skb_headroom(skb) < TX_OVERHEAD) {
-		struct sk_buff *skb2;
-
-		skb2 = skb_copy_expand(skb, TX_OVERHEAD, 0, flags);
+	if (skb_cow_head(skb, TX_OVERHEAD)) {
 		dev_kfree_skb_any(skb);
-		skb = skb2;
-		if (!skb)
-			return NULL;
+		return NULL;
 	}
 
 	if (lan78xx_linearize(skb) < 0)