netfilter: nat: fix cmp return value

The comparator works like memcmp, i.e. 0 means objects are equal.
In other words, when objects are distinct they are treated as identical,
when they are distinct they are allegedly the same.

The first case is rare (distinct objects are unlikely to get hashed to
same bucket).

The second case results in unneeded port conflict resolutions attempts.

Fixes: 870190a9 ("netfilter: nat: convert nat bysrc hash to rhashtable")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_core.c

@@ -193,9 +193,12 @@ static int nf_nat_bysource_cmp(struct rhashtable_compare_arg *arg,
 	const struct nf_nat_conn_key *key = arg->key;
 	const struct nf_conn *ct = obj;
 
-	return same_src(ct, key->tuple) &&
-	       net_eq(nf_ct_net(ct), key->net) &&
-	       nf_ct_zone_equal(ct, key->zone, IP_CT_DIR_ORIGINAL);
+	if (!same_src(ct, key->tuple) ||
+	    !net_eq(nf_ct_net(ct), key->net) ||
+	    !nf_ct_zone_equal(ct, key->zone, IP_CT_DIR_ORIGINAL))
+		return 1;
+
+	return 0;
 }
 
 static struct rhashtable_params nf_nat_bysource_params = {