ovn-northd: logical router icmp response should not care about inport

ovn-northd: logical router icmp response should not care about inport

When responding to icmp echo requests (aka ping) packets, the logical
router should not restrict responses based on the inport.

Example diagram:

vm: IP1.1 (subnet1)
logical_router: IP1.2 (subnet1) and IP2.2 (subnet2)

   vm -------[subnet1]------- logical_router -------[subnet2]
   <IP1.1>                <IP1.2>        <IP2.2>

vm should be able to ping <IP2.2>, even though it is an address
of a subnet that can only be reached through L3 routing.

Reference to the mailing list thread:
http://openvswitch.org/pipermail/discuss/2016-May/021172.html

Signed-off-by: Ben Pfaff <blp@ovn.org>