netfilter: add new hook nfnl subsystem

This nfnl subsystem allows to dump the list of all active netfiler hooks,
e.g. defrag, conntrack, nf/ip/arp/ip6tables and so on.

This helps to see what kind of features are currently enabled in
the network stack.

Sample output from nft tool using this infra:

 $ nft list hook ip input
 family ip hook input {
   +0000000010 nft_do_chain_inet [nf_tables] # nft table firewalld INPUT
   +0000000100 nf_nat_ipv4_local_in [nf_nat]
   +2147483647 ipv4_confirm [nf_conntrack]
 }

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink.c

@@ -68,6 +68,7 @@ static const char *const nfnl_lockdep_names[NFNL_SUBSYS_COUNT] = {
 	[NFNL_SUBSYS_CTHELPER] = "nfnl_subsys_cthelper",
 	[NFNL_SUBSYS_NFTABLES] = "nfnl_subsys_nftables",
 	[NFNL_SUBSYS_NFT_COMPAT] = "nfnl_subsys_nftcompat",
+	[NFNL_SUBSYS_HOOK] = "nfnl_subsys_hook",
 };
 
 static const int nfnl_group2type[NFNLGRP_MAX+1] = {