Fix ICMP on concrete router interface

[Dima Kuznetsov]

Disabling L3 agent breaks at test accessing concrete router interface.

L3 app has code that handles packets bound to router interfaces, it forwards those packets to the controller, which generates a response and sends them back.

The code above relies on setting reg7 to port key of router interface, but L2 app does not handle router interfaces.
We should have L2 app take into account those ports