Synchronise the smoothed metric when metric is infinite.

When a route is retracted, we want to switch to an alternate route
straight away even if the smoothed metric hasn't converged yet.
Setting the smoothed metric to infinity in this case makes this happen.

route.c

@@ -611,10 +611,10 @@ route_smoothed_metric(struct babel_route *route)
 {
     int metric = route_metric(route);
 
-    if(smoothing_half_life <= 0 ||
-       /* Protect against clock stepping. */
-       route->smoothed_metric_time > now.tv_sec ||
-       route->smoothed_metric == metric) {
+    if(smoothing_half_life <= 0 ||                 /* no smoothing */
+       metric >= INFINITY ||                       /* route retracted */
+       route->smoothed_metric_time > now.tv_sec || /* clock stepped */
+       route->smoothed_metric == metric) {         /* already converged */
         route->smoothed_metric = metric;
         route->smoothed_metric_time = now.tv_sec;
     } else {