cmd/gofmt: update error handling when writing to backup file

As per commit aa0ae755, handling of io.ErrShortWrite is done in
*File.Write() itself.

Change-Id: I92924b51e8df2ae88e6e50318348f44973addba8
Reviewed-on: https://go-review.googlesource.com/113696Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/gofmt/gofmt.go

@@ -319,10 +319,7 @@ func backupFile(filename string, data []byte, perm os.FileMode) (string, error)
 	}
 
 	// write data to backup file
-	n, err := f.Write(data)
-	if err == nil && n < len(data) {
-		err = io.ErrShortWrite
-	}
+	_, err = f.Write(data)
 	if err1 := f.Close(); err == nil {
 		err = err1
 	}