netfilter: nf_queue: enqueue skbs with NULL dst

Bridge packets that are forwarded have skb->dst == NULL and get
dropped by the check introduced by
b60a7738 (net: make skb_dst_force
return true when dst is refcounted).

To fix this we check skb_dst() before skb_dst_force(), so we don't
drop skb packet with dst == NULL. This holds also for skb at the
PRE_ROUTING hook so we remove the second check.

Fixes: b60a7738 ("net: make skb_dst_force return true when dst is refcounted")
Signed-off-by: Marco Oliverio <marco.oliverio@tanaza.com>
Signed-off-by: Rocco Folino <rocco.folino@tanaza.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_queue.c

@@ -189,7 +189,7 @@ static int __nf_queue(struct sk_buff *skb, const struct nf_hook_state *state,
 		goto err;
 	}
 
-	if (!skb_dst_force(skb) && state->hook != NF_INET_PRE_ROUTING) {
+	if (skb_dst(skb) && !skb_dst_force(skb)) {
 		status = -ENETDOWN;
 		goto err;
 	}