netfilter: nf_tables: relax string validation of NFTA_CHAIN_TYPE

Use NLA_STRING for consistency with other string attributes in
nf_tables.
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -541,7 +541,7 @@ static const struct nla_policy nft_chain_policy[NFTA_CHAIN_MAX + 1] = {
 				    .len = NFT_CHAIN_MAXNAMELEN - 1 },
 	[NFTA_CHAIN_HOOK]	= { .type = NLA_NESTED },
 	[NFTA_CHAIN_POLICY]	= { .type = NLA_U32 },
-	[NFTA_CHAIN_TYPE]	= { .type = NLA_NUL_STRING },
+	[NFTA_CHAIN_TYPE]	= { .type = NLA_STRING },
 	[NFTA_CHAIN_COUNTERS]	= { .type = NLA_NESTED },
 };