xtables: SET target: Add mapping of meta informations (skbinfo ipset extension)

xtables: SET target: Add mapping of meta informations (skbinfo ipset extension)

This feature add support of mapping metainformation to packets like nftables maps or
ipfw tables. Currently we can map firewall mark, tc priority and hardware NIC queue.
Usage of this functionality allowed only from mangle table. We can map tc priority
only in OUTPUT/FORWARD/POSTROUTING chains because it rewrite by route decision.
If entry doesn't exist in the set nothing of fields changed.

Example of classify by destination address:
iptables -t mangle -A POSTROUTING -o eth0 -j SET --map-set DST2CLASS dst --map-prio

Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>