netfilter: nf_tables: add NFT_CHAIN_POLICY_UNSET and use it

Default policy is defined as a unsigned 8-bit field, do not use a
negative value to leave it unset, use this new NFT_CHAIN_POLICY_UNSET
instead.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1715,7 +1715,7 @@ static int nf_tables_addchain(struct nft_ctx *ctx, u8 family, u8 genmask,
 		goto err2;
 	}
 
-	nft_trans_chain_policy(trans) = -1;
+	nft_trans_chain_policy(trans) = NFT_CHAIN_POLICY_UNSET;
 	if (nft_is_base_chain(chain))
 		nft_trans_chain_policy(trans) = policy;