net: openvswitch: add meter drop reason

By using an independent drop reason it makes it easy to distinguish
between QoS-triggered or flow-triggered drop.

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>