Return minimum disk size field from snapshot response

We incorrectly returned the billable size of the snapshot instead of the
minimum size. The latter represents the usable size the volume was
created with and serves as a hint to Kubernetes for how big volumes
created off of snapshots should be.

driver/controller.go

@@ -1108,7 +1108,7 @@ func toCSISnapshot(snap *godo.Snapshot) (*csi.Snapshot, error) {
 	return &csi.Snapshot{
 		SnapshotId:     snap.ID,
 		SourceVolumeId: snap.ResourceID,
-		SizeBytes:      int64(snap.SizeGigaBytes) * giB,
+		SizeBytes:      int64(snap.MinDiskSize) * giB,
 		CreationTime:   tstamp,
 		ReadyToUse:     true,
 	}, nil