sr9700: 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: c9b37458 ("USB2NET : SR9700 : One chip USB 1.1 USB2NET SR9700Device Driver Support")
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/sr9700.c

@@ -456,14 +456,9 @@ static struct sk_buff *sr9700_tx_fixup(struct usbnet *dev, struct sk_buff *skb,
 
 	len = skb->len;
 
-	if (skb_headroom(skb) < SR_TX_OVERHEAD) {
-		struct sk_buff *skb2;
-
-		skb2 = skb_copy_expand(skb, SR_TX_OVERHEAD, 0, flags);
+	if (skb_cow_head(skb, SR_TX_OVERHEAD)) {
 		dev_kfree_skb_any(skb);
-		skb = skb2;
-		if (!skb)
-			return NULL;
+		return NULL;
 	}
 
 	__skb_push(skb, SR_TX_OVERHEAD);