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: Eric Dumazet <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_netem.c

···
 	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);
 	}