netfilter: nfnetlink_hook: Dump flowtable info

Introduce NFNL_HOOK_TYPE_NFT_FLOWTABLE to distinguish flowtable hooks
from base chain ones. Nested attributes are shared with the old NFTABLES
hook info type since they fit apart from their misleading name.

Old nftables in user space will ignore this new hook type and thus
continue to print flowtable hooks just like before, e.g.:

| family netdev {
| 	hook ingress device test0 {
| 		 0000000000 nf_flow_offload_ip_hook [nf_flow_table]
| 	}
| }

With this patch in place and support for the new hook info type, output
becomes more useful:

| family netdev {
| 	hook ingress device test0 {
| 		 0000000000 flowtable ip mytable myft [nf_flow_table]
| 	}
| }

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nfnetlink_hook.h

@@ -61,10 +61,12 @@ enum nfnl_hook_chain_desc_attributes {
  *
  * @NFNL_HOOK_TYPE_NFTABLES: nf_tables base chain
  * @NFNL_HOOK_TYPE_BPF: bpf program
+ * @NFNL_HOOK_TYPE_NFT_FLOWTABLE: nf_tables flowtable
  */
 enum nfnl_hook_chaintype {
 	NFNL_HOOK_TYPE_NFTABLES = 0x1,
 	NFNL_HOOK_TYPE_BPF,
+	NFNL_HOOK_TYPE_NFT_FLOWTABLE,
 };
 
 /**