vfs: fix saving from chrome without --vfs-cache-mode writes

Due to Chrome's rather complicated use of file handles when saving
files from the download windows, rclone was attempting to truncate a
closed file.

The file appeared closed due to the handling of 0 length files.

This patch removes the check for the file being closed in the
WriteFileHandle.Truncate call. This is safe because the only action
this method takes is to emit an error message if the file is the wrong
size.

See: https://forum.rclone.org/t/google-drive-cannot-save-files-directly-from-browser-to-gdrive-mounted-path/17992/

vfs/write.go

@@ -283,11 +283,9 @@ func (fh *WriteFileHandle) Stat() (os.FileInfo, error) {
 
 // Truncate file to given size
 func (fh *WriteFileHandle) Truncate(size int64) (err error) {
+	// defer log.Trace(fh.remote, "size=%d", size)("err=%v", &err)
 	fh.mu.Lock()
 	defer fh.mu.Unlock()
-	if fh.closed {
-		return ECLOSED
-	}
 	if size != fh.offset {
 		fs.Errorf(fh.remote, "WriteFileHandle: Truncate: Can't change size without --vfs-cache-mode >= writes")
 		return EPERM