netfilter: nft_nat: allow to specify layer 4 protocol NAT only

nft_nat reports a bogus EAFNOSUPPORT if no layer 3 information is specified.

Fixes: d07db988 ("netfilter: nf_tables: introduce nft_validate_register_load()")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>