netfilter: nf_tables: Introduce new 64-bit helper register functions

Introduce new helper functions to load/store 64-bit values onto/from
registers:

 - nft_reg_store64
 - nft_reg_load64

This commit also re-orders all these helpers from smallest to largest
target bit size.

Signed-off-by: Ander Juaristi <a@juaristi.eus>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>