File must be sync'd before closing (#7657)

- group sync and close action into a single defer statement to avoid
  evaluation order related bugs in future.

cmd/posix.go

@@ -1182,8 +1182,10 @@ func (s *posix) CreateFile(volume, path string, fileSize int64, r io.Reader) (er
 			return err
 		}
 	}
-	defer w.Sync() // Sync before close.
+	defer func() {
-	defer w.Close()
+		w.Sync() // Sync before close.
+		w.Close()
+	}()
 
 	var e error
 	if fileSize > 0 {