netfilter: nf_tables: get rid of pernet families

Now that we have a single table list for each netns, we can get rid of
one pointer per family and the global afinfo list, thus, shrinking
struct netns for nftables that now becomes 64 bytes smaller.

And call __nft_release_afinfo() from __net_exit path accordingly to
release netnamespace objects on removal.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>