netfilter: nft_hash: rename nft_hash to nft_jhash

This patch renames the local nft_hash structure and functions
to nft_jhash in order to prepare the nft_hash module code to
add new hash functions.

Signed-off-by: Laura Garcia Liebana <laura.garcia@zevenet.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>