net_sched: properly check for empty skb array on error path

First, the check of &q->ring.queue against NULL is wrong, it
is always false. We should check the value rather than the address.

Secondly, we need the same check in pfifo_fast_reset() too,
as both ->reset() and ->destroy() are called in qdisc_destroy().

Fixes: c5ad119f ("net: sched: pfifo_fast use skb_array")
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -659,6 +659,12 @@ static void pfifo_fast_reset(struct Qdisc *qdisc)
 		struct skb_array *q = band2list(priv, band);
 		struct sk_buff *skb;
 
+		/* NULL ring is possible if destroy path is due to a failed
+		 * skb_array_init() in pfifo_fast_init() case.
+		 */
+		if (!q->ring.queue)
+			continue;
+
 		while ((skb = skb_array_consume_bh(q)) != NULL)
 			kfree_skb(skb);
 	}
@@ -719,7 +725,7 @@ static void pfifo_fast_destroy(struct Qdisc *sch)
 		/* NULL ring is possible if destroy path is due to a failed
 		 * skb_array_init() in pfifo_fast_init() case.
 		 */
-		if (!&q->ring.queue)
+		if (!q->ring.queue)
 			continue;
 		/* Destroy ring but no need to kfree_skb because a call to
 		 * pfifo_fast_reset() has already done that work.