netfilter: nft_hash: add missing NFTA_HASH_OFFSET's nla_policy

Missing the nla_policy description will also miss the validation check
in kernel.

Fixes: 70ca767e ("netfilter: nft_hash: Add hash offset value")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>