netfilter: nf_tables: fix wrong datatype in nft_validate_data_load()

This patch fixes dictionary mappings, eg.

 add rule ip filter input meta dnat set tcp dport map { 22 => 1.1.1.1, 23 => 2.2.2.2 }

The kernel was returning -EINVAL in nft_validate_data_load() since
the type of the set element data that is passed was the real userspace
datatype instead of NFT_DATA_VALUE.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>