netfilter: nft_tproxy: restrict to prerouting hook

TPROXY is only allowed from prerouting, but nft_tproxy doesn't check this.
This fixes a crash (null dereference) when using tproxy from e.g. output.

Fixes: 4ed8eb6570 ("netfilter: nf_tables: Add native tproxy support")
Reported-by: Shell Chen <xierch@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nft_tproxy.c

@@ -312,6 +312,13 @@ static int nft_tproxy_dump(struct sk_buff *skb,
 	return 0;
 }
 
+static int nft_tproxy_validate(const struct nft_ctx *ctx,
+			       const struct nft_expr *expr,
+			       const struct nft_data **data)
+{
+	return nft_chain_validate_hooks(ctx->chain, 1 << NF_INET_PRE_ROUTING);
+}
+
 static struct nft_expr_type nft_tproxy_type;
 static const struct nft_expr_ops nft_tproxy_ops = {
 	.type		= &nft_tproxy_type,
@@ -321,6 +328,7 @@ static const struct nft_expr_ops nft_tproxy_ops = {
 	.destroy	= nft_tproxy_destroy,
 	.dump		= nft_tproxy_dump,
 	.reduce		= NFT_REDUCE_READONLY,
+	.validate	= nft_tproxy_validate,
 };
 
 static struct nft_expr_type nft_tproxy_type __read_mostly = {