netfilter: meta: add support for setting skb->pkttype

This allows to redirect bridged packets to local machine:

ether type ip ether daddr set aa:53:08:12:34:56 meta pkttype set unicast
Without 'set unicast', ip stack discards PACKET_OTHERHOST skbs.

It is also useful to add support for a '-m cluster like' nft rule
(where switch floods packets to several nodes, and each cluster node
 node processes a subset of packets for load distribution).

Mangling is restricted to HOST/OTHER/BROAD/MULTICAST, i.e. you cannot set
skb->pkt_type to PACKET_KERNEL or change PACKET_LOOPBACK to PACKET_HOST.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>