netfilter: nf_tables: check NFT_SET_CONCAT flag if field_count is specified

Since f3a2181e ("netfilter: nf_tables: Support for sets with
multiple ranged fields"), it possible to combine intervals and
concatenations. Later on, ef516e86 ("netfilter: nf_tables:
reintroduce the NFT_SET_CONCAT flag") provides the NFT_SET_CONCAT flag
for userspace to report that the set stores a concatenation.

Make sure NFT_SET_CONCAT is set on if field_count is specified for
consistency. Otherwise, if NFT_SET_CONCAT is specified with no
field_count, bail out with EINVAL.

Fixes: ef516e86

 ("netfilter: nf_tables: reintroduce the NFT_SET_CONCAT flag")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>