pppoe: Fix socket leak.

Move SKB trim before we lookup the socket so we don't have to
put it on failure.

Based upon an initial patch by Jarek Poplawski and suggestions
from Herbert Xu.

Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppoe.c

@@ -399,11 +399,11 @@ static int pppoe_rcv(struct sk_buff *skb,
 	if (skb->len < len)
 		goto drop;
 
-	po = get_item(ph->sid, eth_hdr(skb)->h_source, dev->ifindex);
-	if (!po)
+	if (pskb_trim_rcsum(skb, len))
 		goto drop;
 
-	if (pskb_trim_rcsum(skb, len))
+	po = get_item(ph->sid, eth_hdr(skb)->h_source, dev->ifindex);
+	if (!po)
 		goto drop;
 
 	return sk_receive_skb(sk_pppox(po), skb, 0);