netfilter: nft_nat: return EOPNOTSUPP if type or flags are not supported

[ Upstream commit 0d7c83463fdf7841350f37960a7abadd3e650b41 ]

Instead of EINVAL which should be used for malformed netlink messages.

Fixes: eb31628e ("netfilter: nf_tables: Add support for IPv6 NAT")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>