netfilter: nf_tables: export set count and backend name to userspace

nf_tables picks a suitable set backend implementation (bitmap, hash,
rbtree..) based on the userspace requirements.

Figuring out the chosen backend requires information about the set flags
and the kernel version.  Export this to userspace so nft can include this
information in '--debug=netlink' output.

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