Make the vsc created by backup sync controller deletable

Fixes #4760

This commit make changes in 2 parts:
1) When a volumesnapshotcontent is persisted during backup, velero will reset its
   `Source` field to remove the VolumeHandle, so that the
   csi-snapshotter will not try to call `CreateSnapshot` when its synced
   to another cluster with a backup.
2) Make sure the referenced volumesnapshotclasses are persisted and
   synced with the backup, so that when the volumesnapshotcontent is
   deleted the storage snapshot is also removed.

Signed-off-by: Daniel Jiang <jiangd@vmware.com>

pkg/controller/backup_sync_controller.go

@@ -283,6 +283,22 @@ func (c *backupSyncController) run() {
 			if features.IsEnabled(velerov1api.CSIFeatureFlag) {
 				// we are syncing these objects only to ensure that the storage snapshots are cleaned up
 				// on backup deletion or expiry.
+				log.Info("Syncing CSI volumesnapshotclasses in backup")
+				vsClasses, err := backupStore.GetCSIVolumeSnapshotClasses(backupName)
+				if err != nil {
+					log.WithError(errors.WithStack(err)).Error("Error getting CSI volumesnapclasses for this backup from backup store")
+					continue
+				}
+				for _, vsClass := range vsClasses {
+					vsClass.ResourceVersion = ""
+					created, err := c.csiSnapshotClient.SnapshotV1().VolumeSnapshotClasses().Create(context.TODO(), vsClass, metav1.CreateOptions{})
+					if err != nil {
+						log.WithError(errors.WithStack(err)).Errorf("Error syncing volumesnapshotclass %s into cluster", vsClass.Name)
+						continue
+					}
+					log.Infof("Created CSI volumesnapshotclass %s", created.Name)
+				}
+
 				log.Info("Syncing CSI volumesnapshotcontents in backup")
 				snapConts, err := backupStore.GetCSIVolumeSnapshotContents(backupName)
 				if err != nil {