netem: fix possible skb leak

skb_checksum_help(skb) can return an error, we must free skb in this
case. qdisc_drop(skb, sch) can also be feeded with a NULL skb (if
skb_unshare() failed), so lets use this generic helper.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_netem.c

@@ -408,10 +408,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	if (q->corrupt && q->corrupt >= get_crandom(&q->corrupt_cor)) {
 		if (!(skb = skb_unshare(skb, GFP_ATOMIC)) ||
 		    (skb->ip_summed == CHECKSUM_PARTIAL &&
-		     skb_checksum_help(skb))) {
-			sch->qstats.drops++;
-			return NET_XMIT_DROP;
-		}
+		     skb_checksum_help(skb)))
+			return qdisc_drop(skb, sch);
 
 		skb->data[net_random() % skb_headlen(skb)] ^= 1<<(net_random() % 8);
 	}