net_sched: netem: do not call qdisc_drop() with a NULL skb

If skb_unshare() fails, we call qdisc_drop() with a NULL skb, which
is no longer supported.

Fixes: 520ac30f ("net_sched: drop packets after root qdisc lock is released")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_netem.c

@@ -487,10 +487,14 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		skb = segs;
 		segs = segs->next;
 
-		if (!(skb = skb_unshare(skb, GFP_ATOMIC)) ||
-		    (skb->ip_summed == CHECKSUM_PARTIAL &&
-		     skb_checksum_help(skb))) {
-			rc = qdisc_drop(skb, sch, to_free);
+		skb = skb_unshare(skb, GFP_ATOMIC);
+		if (unlikely(!skb)) {
+			qdisc_qstats_drop(sch);
+			goto finish_segs;
+		}
+		if (skb->ip_summed == CHECKSUM_PARTIAL &&
+		    skb_checksum_help(skb)) {
+			qdisc_drop(skb, sch, to_free);
 			goto finish_segs;
 		}