netfilter: nft_meta: add pkttype support

Add pkttype support for ip, ipv6 and inet families of tables.

This allows you to fetch the meta packet type based on the link layer
information. The loopback traffic is a special case, the packet type
is guessed from the network layer header.

No special handling for bridge and arp since we're not going to see
such traffic in the loopback interface.

Joint work with Alvaro Neira Ayuso <alvaroneay@gmail.com>

Signed-off-by: Alvaro Neira Ayuso <alvaroneay@gmail.com>
Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>