netfilter: nftables: add nft_parse_register_load() and use it

netfilter: nftables: add nft_parse_register_load() and use it

[ 4bcefa21f9c3522540a0bfa242b9511210e8ada8 ]

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

This update requires to replace:

	enum nft_registers      sreg:8;

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

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

when passing the register field as reference.

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