netfilter: nft_nat: correct length for loading protocol registers

The values in the protocol registers are two bytes wide.  However, when
parsing the register loads, the code currently uses the larger 16-byte
size of a `union nf_inet_addr`.  Change it to use the (correct) size of
a `union nf_conntrack_man_proto` instead.

Fixes: d07db988 ("netfilter: nf_tables: introduce nft_validate_register_load()")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_nat.c

@@ -226,7 +226,7 @@ static int nft_nat_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		priv->flags |= NF_NAT_RANGE_MAP_IPS;
 	}
 
-	plen = sizeof_field(struct nf_nat_range, min_addr.all);
+	plen = sizeof_field(struct nf_nat_range, min_proto.all);
 	if (tb[NFTA_NAT_REG_PROTO_MIN]) {
 		err = nft_parse_register_load(tb[NFTA_NAT_REG_PROTO_MIN],
 					      &priv->sreg_proto_min, plen);