netfilter: nf_tables: nft_meta module get/set ops

This patch adds kernel support for the meta expression in get/set
flavour. The set operation indicates that a given packet has to be
set with a property, currently one of mark, priority, nftrace.
The get op is what was currently working: evaluate the given
packet property.

In the nftrace case, the value is always 1. Such behaviour is copied
from net/netfilter/xt_TRACE.c

The NFTA_META_DREG and NFTA_META_SREG attributes are mutually
exclusives.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>