nftables: Convert nft_hash to use generic rhashtable

The sizing of the hash table and the practice of requiring a lookup
to retrieve the pprev to be stored in the element cookie before the
deletion of an entry is left intact.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Patrick McHardy <kaber@trash.net>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>