smsc75xx: use skb_cow_head() to deal with cloned skbs

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: d0cad871 ("smsc75xx: SMSC LAN75xx USB gigabit ethernet adapter driver")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: James Hughes <james.hughes@raspberrypi.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/smsc75xx.c

@@ -2203,12 +2203,8 @@ static struct sk_buff *smsc75xx_tx_fixup(struct usbnet *dev,
 {
 	u32 tx_cmd_a, tx_cmd_b;
 
-	if (skb_headroom(skb) < SMSC75XX_TX_OVERHEAD) {
-		struct sk_buff *skb2 =
-			skb_copy_expand(skb, SMSC75XX_TX_OVERHEAD, 0, flags);
+	if (skb_cow_head(skb, SMSC75XX_TX_OVERHEAD)) {
 		dev_kfree_skb_any(skb);
-		skb = skb2;
-		if (!skb)
 		return NULL;
 	}