netfilter: nf_tables: merge ipv4 and ipv6 nat chain types

Merge the ipv4 and ipv6 nat chain type. This is the last
missing piece which allows to provide inet family support
for nat in a follow patch.

The kconfig knobs for ipv4/ipv6 nat chain are removed, the
nat chain type will be built unconditionally if NFT_NAT
expression is enabled.

Before:
   text	   data	    bss	    dec	    hex	filename
   1576     896       0    2472     9a8 nft_chain_nat_ipv4.ko
   1697     896       0    2593     a21 nft_chain_nat_ipv6.ko

After:
   text	   data	    bss	    dec	    hex	filename
   1832     896       0    2728     aa8 nft_chain_nat.ko

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