Fix nil pointer issue when getting metrics from volume mounter

Currently it is possible that the mounter object stored in Mounted
Volume data structure in the actual state of kubelet volume manager is
nil if this information is recovered from state sync process. This will
cause nil pointer issue when calculating stats in volume_stat_calculator.
A quick fix is to not return the volume if its mounter is nil. A more
complete fix is to also recover mounter object when reconstructing the
volume data structure which will be addressed in PR kubernetes#33616

pkg/kubelet/kubelet_volumes.go

@@ -40,6 +40,12 @@ func (kl *Kubelet) ListVolumesForPod(podUID types.UID) (map[string]volume.Volume
 	podVolumes := kl.volumeManager.GetMountedVolumesForPod(
 		volumetypes.UniquePodName(podUID))
 	for outerVolumeSpecName, volume := range podVolumes {
+		// TODO: volume.Mounter could be nil if volume object is recovered
+		// from reconciler's sync state process. PR 33616 will fix this problem
+		// to create Mounter object when recovering volume state.
+		if volume.Mounter == nil {
+			continue
+		}
 		volumesToReturn[outerVolumeSpecName] = volume.Mounter
 	}