netfilter: nf_tables: add hash expression

This patch adds a new hash expression, this provides jhash support but
this can be extended to support for other hash functions. The modulus
and seed already comes embedded into this new expression.

Use case example:

	... meta mark set hash ip saddr mod 10

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