ipv4: Flush per-ns routing cache more sanely.

Flush the routing cache only of entries that match the
network namespace in which the purge event occurred.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

net/ipv4/fib_frontend.c

@@ -987,7 +987,11 @@ static int fib_netdev_event(struct notifier_block *this, unsigned long event, vo
 		rt_cache_flush(dev_net(dev), 0);
 		break;
 	case NETDEV_UNREGISTER_BATCH:
-		rt_cache_flush_batch();
+		/* The batch unregister is only called on the first
+		 * device in the list of devices being unregistered.
+		 * Therefore we should not pass dev_net(dev) in here.
+		 */
+		rt_cache_flush_batch(NULL);
 		break;
 	}
 	return NOTIFY_DONE;