Fix incorrect test for clock stepping in route_smoothed_metric.
Fix incorrect test for clock stepping in route_smoothed_metric.

This would cause the smoothed metric to instantaneously jump to
the true metric when route_smoothed_metric was called twice in the
same second.

tcatm/babeld,jech/babeld,jech/babeld,Gwendocg/babeldToS,Gwendocg/babeldToS,wlanslovenija/babeld,woniullb/babeld,sudomesh/babeld,wlanslovenija/babeld,dtaht/babeld-shortrtt-metrics,Drooids/babeld,boutier/babeld