netfilter: nft_ct: sanitize layer 3 and 4 protocol number in custom expectations

- Disallow families other than NFPROTO_{IPV4,IPV6,INET}.
- Disallow layer 4 protocol with no ports, since destination port is a
  mandatory attribute for this object.

Fixes: 857b4602 ("netfilter: nft_ct: add ct expectations support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>