netfilter: nfqueue: don't use prev pointer

Usage of -prev seems buggy.  While packet was out our hook cannot be
removed but we have no way to know if the previous one is still valid.

So better not use ->prev at all.  Since NF_REPEAT just asks to invoke
same hook function again, just do so, and continue with nf_interate
if we get an ACCEPT verdict.

A side effect of this change is that if nf_reinject(NF_REPEAT) causes
another REPEAT we will now drop the skb instead of a kernel loop.

However, NF_REPEAT loops would be a bug so this should not happen anyway.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_queue.c

@@ -192,10 +192,8 @@ void nf_reinject(struct nf_queue_entry *entry, unsigned int verdict)
 	nf_queue_entry_release_refs(entry);
 
 	/* Continue traversal iff userspace said ok... */
-	if (verdict == NF_REPEAT) {
-		elem = list_entry(elem->list.prev, struct nf_hook_ops, list);
-		verdict = NF_ACCEPT;
-	}
+	if (verdict == NF_REPEAT)
+		verdict = elem->hook(elem->priv, skb, &entry->state);
 
 	if (verdict == NF_ACCEPT) {
 		afinfo = nf_get_afinfo(entry->state.pf);