Fix memory not being unmapped

weed/storage/volume_read_write.go

@@ -49,10 +49,10 @@ func (v *Volume) Destroy() (err error) {
 		err = fmt.Errorf("volume %d is compacting", v.Id)
 		return
 	}
-	mem_map, exists := memory_map.FileMemoryMap[v.FileName()]
+	mem_map, exists := memory_map.FileMemoryMap[v.dataFile.Name()]
 	if exists {
 		mem_map.DeleteFileAndMemoryMap()
-		delete(memory_map.FileMemoryMap, v.FileName())
+		delete(memory_map.FileMemoryMap, v.dataFile.Name())
 	}
 
 	v.Close()