netfilter: nftables: add nft_parse_register_store() and use it

[ Upstream commit 345023b0 ]

This new function combines the netlink register attribute parser
and the store validation function.

This update requires to replace:

        enum nft_registers      dreg:8;

in many of the expression private areas otherwise compiler complains
with:

        error: cannot take address of bit-field ‘dreg’

when passing the register field as reference.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>