netfilter: nft_nat: allow to specify layer 4 protocol NAT only

nft_nat reports a bogus EAFNOSUPPORT if no layer 3 information is specified.

Fixes: d07db9884a ("netfilter: nf_tables: introduce nft_validate_register_load()")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_nat.c

@@ -201,7 +201,9 @@ static int nft_nat_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		alen = sizeof_field(struct nf_nat_range, min_addr.ip6);
 		break;
 	default:
-		return -EAFNOSUPPORT;
+		if (tb[NFTA_NAT_REG_ADDR_MIN])
+			return -EAFNOSUPPORT;
+		break;
 	}
 	priv->family = family;