netfilter: nft_ct: add NFT_CT_{SRC,DST}_{IP,IP6}

All existing keys, except the NFT_CT_SRC and NFT_CT_DST are assumed to
have strict datatypes. This is causing problems with sets and
concatenations given the specific length of these keys is not known.

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