netfilter: nf_tables: reject NFT_SET_CONCAT with not field length description

[ Upstream commit 113661e07460a6604aacc8ae1b23695a89e7d4b3 ]

It is still possible to set on the NFT_SET_CONCAT flag by specifying a
set size and no field description, report EINVAL in such case.

Fixes: 1b6345d4 ("netfilter: nf_tables: check NFT_SET_CONCAT flag if field_count is specified")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>