netfilter: nf_nat: mask out non-verdict bits when checking return value

Same as previous change: we need to mask out the non-verdict bits, as
upcoming patches may embed an errno value in NF_STOLEN verdicts too.

NF_DROP could already do this, but not all called functions do this.

Checks that only test ret vs NF_ACCEPT are fine, the 'errno parts'
are always 0 for those.
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nf_nat_proto.c

@@ -999,11 +999,12 @@ static unsigned int
 nf_nat_ipv6_in(void *priv, struct sk_buff *skb,
 	       const struct nf_hook_state *state)
 {
-	unsigned int ret;
+	unsigned int ret, verdict;
 	struct in6_addr daddr = ipv6_hdr(skb)->daddr;
 
 	ret = nf_nat_ipv6_fn(priv, skb, state);
-	if (ret != NF_DROP && ret != NF_STOLEN &&
+	verdict = ret & NF_VERDICT_MASK;
+	if (verdict != NF_DROP && verdict != NF_STOLEN &&
 	    ipv6_addr_cmp(&daddr, &ipv6_hdr(skb)->daddr))
 		skb_dst_drop(skb);