pppoe: Unshare skb before anything else

We need to unshare the skb first as otherwise pskb_may_pull may
write to a shared skb which could be bad.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppoe.c

@@ -432,12 +432,12 @@ static int pppoe_disc_rcv(struct sk_buff *skb,
 	if (dev_net(dev) != &init_net)
 		goto abort;
 
-	if (!pskb_may_pull(skb, sizeof(struct pppoe_hdr)))
-		goto abort;
-
 	if (!(skb = skb_share_check(skb, GFP_ATOMIC)))
 		goto out;
 
+	if (!pskb_may_pull(skb, sizeof(struct pppoe_hdr)))
+		goto abort;
+
 	ph = pppoe_hdr(skb);
 	if (ph->code != PADT_CODE)
 		goto abort;