netfilter: nft_hash: add map lookups for hashing operations

This patch creates new attributes to accept a map as argument and
then perform the lookup with the generated hash accordingly.

Both current hash functions are supported: Jenkins and Symmetric Hash.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>