Revert "The lock should not be released immediately after it is acquired" (#32)

* Revert "The lock should not be released immediately after it is acquired (#29)"

This reverts commit e5f0c695.

* Update writer.go

pkg/vfs/writer.go

@@ -355,11 +355,7 @@ func (f *fileWriter) Flush(ctx meta.Context) syscall.Errno {
 }
 
 func (f *fileWriter) Close(ctx meta.Context) syscall.Errno {
-	defer func() {
-		f.Lock()
-		f.w.free(f)
-		f.Unlock()
-	}()
+	defer f.w.free(f)
 	return f.Flush(ctx)
 }