volume: sync volume file right before compaction

fix https://github.com/chrislusf/seaweedfs/issues/1237

weed/storage/backend/memory_map/memory_map_backend.go

@@ -58,3 +58,7 @@ func (mmf *MemoryMappedFile) GetStat() (datSize int64, modTime time.Time, err er
 func (mmf *MemoryMappedFile) Name() string {
 	return mmf.mm.File.Name()
 }
+
+func (mm *MemoryMappedFile) Sync() error {
+	return nil
+}