netpoll: Use skb_irq_freeable to make zap_completion_queue safe.

Replace the test in zap_completion_queue to test when it is safe to
free skbs in hard irq context with skb_irq_freeable ensuring we only
free skbs when it is safe, and removing the possibility of subtle
problems.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -270,7 +270,7 @@ static void zap_completion_queue(void)
 		while (clist != NULL) {
 			struct sk_buff *skb = clist;
 			clist = clist->next;
-			if (skb->destructor) {
+			if (!skb_irq_freeable(skb)) {
 				atomic_inc(&skb->users);
 				dev_kfree_skb_any(skb); /* put this one back */
 			} else {