netfilter: nf_tables: add hardware offload support

This patch adds hardware offload support for nftables through the
existing netdev_ops->ndo_setup_tc() interface, the TC_SETUP_CLSFLOWER
classifier and the flow rule API. This hardware offload support is
available for the NFPROTO_NETDEV family and the ingress hook.

Each nftables expression has a new ->offload interface, that is used to
populate the flow rule object that is attached to the transaction
object.

There is a new per-table NFT_TABLE_F_HW flag, that is set on to offload
an entire table, including all of its chains.

This patch supports for basic metadata (layer 3 and 4 protocol numbers),
5-tuple payload matching and the accept/drop actions; this also includes
basechain hardware offload only.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>