// check if m3u8 file ready
// *STAGE* 3 for transcoding
// transcoder : create via create_transcode
// m3u8file : m3u8 file, the format is xxx.mkv_1920x1080.m3u8. base on media_file_path_symlink_name
// regen : remove m3u8 file at cache and then regenerate
// return : >=0 for ok, failed otherwise