blackhole_netdev: use blackhole_netdev to invalidate dst entries

Use blackhole_netdev instead of 'lo' device with lower MTU when marking
dst "dead".

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Tested-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -160,7 +160,7 @@ void dst_dev_put(struct dst_entry *dst)
 		dst->ops->ifdown(dst, dev, true);
 	dst->input = dst_discard;
 	dst->output = dst_discard_out;
-	dst->dev = dev_net(dst->dev)->loopback_dev;
+	dst->dev = blackhole_netdev;
 	dev_hold(dst->dev);
 	dev_put(dev);
 }