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

stable inclusion
from linux-4.19.201
commit 1cb5995a39eb3dc97a7539d00d2c82be030e0bb8

--------------------------------

[ Upstream commit a33f387e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>