netfilter: nf_tables: add __nft_chain_type_get()

This new helper function validates that unknown family and chain type
coming from userspace do not trigger an out-of-bound array access. Bail
out in case __nft_chain_type_get() returns NULL from
nft_chain_parse_hook().

Fixes: 9370761c ("netfilter: nf_tables: convert built-in tables/chains to chain types")
Reported-by:  <syzbot+156a04714799b1d480bc@syzkaller.appspotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>