netfilter: nf_tables: make sets built-in

Placing nftables set support in an extra module is pointless:

1. nf_tables needs dynamic registeration interface for sake of one module
2. nft heavily relies on sets, e.g. even simple rule like
   "nft ... tcp dport { 80, 443 }" will not work with _SETS=n.

IOW, either nftables isn't used or both nf_tables and nf_tables_set
modules are needed anyway.

With extra module:
 307K net/netfilter/nf_tables.ko
  79K net/netfilter/nf_tables_set.ko

   text  data  bss     dec filename
 146416  3072  545  150033 nf_tables.ko
  35496  1817    0   37313 nf_tables_set.ko

This patch:
 373K net/netfilter/nf_tables.ko

 178563  4049  545  183157 nf_tables.ko

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