openvswitch: Add conntrack limit netlink definition

Define netlink messages and attributes to support user kernel
communication that uses the conntrack limit feature.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>