netfilter: nf_tables: get rid of NFT_REG_VERDICT usage

Replace the array of registers passed to expressions by a struct nft_regs,
containing the verdict as a seperate member, which aliases to the
NFT_REG_VERDICT register.

This is needed to seperate the verdict from the data registers completely,
so their size can be changed.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>