netfilter: nft_hash: convert to use rhashtable callbacks

A following patch will convert sets to use so called set extensions,
where the key is not located in a fixed position anymore. This will
require rhashtable hashing and comparison callbacks to be used.

As preparation, convert nft_hash to use these callbacks without any
functional changes.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>