sched: cls_flower: fix panic on filter replace

The following test case causes a NULL pointer dereference in cls_flower:

  tc filter add dev foo parent 1: flower eth_type ipv4 action ok flowid 1:1
  tc filter replace dev foo parent 1: pref 49152 handle 0x1 \
            flower eth_type ipv6 action ok flowid 1:1

The problem is that commit 77b9900ef53a ("tc: introduce Flower classifier")
accidentally swapped the arguments of list_replace_rcu(), the old
element needs to be the first argument and the new element the second.

Fixes: 77b9900ef53a ("tc: introduce Flower classifier")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_flower.c

@@ -499,7 +499,7 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
 	*arg = (unsigned long) fnew;
 
 	if (fold) {
-		list_replace_rcu(&fnew->list, &fold->list);
+		list_replace_rcu(&fold->list, &fnew->list);
 		tcf_unbind_filter(tp, &fold->res);
 		call_rcu(&fold->rcu, fl_destroy_filter);
 	} else {