[log] Fatalf and Panicf log source is incorrect due to calling Fatal and Panic

[at15]

When handling source, we hard coded to go up to level, which is the caller, but when using Fatalf, it does not call HandleLogWithSource directly, thus the source become logger.go:88 ...

func (l *Logger) Fatal(args ...interface{}) {
	s := fmt.Sprint(args...)
	if !l.source {
		l.h.HandleLog(FatalLevel, time.Now(), s)
	} else {
		l.h.HandleLogWithSource(FatalLevel, time.Now(), s, caller())
	}
	l.h.Flush()
	// TODO: allow user to register hook to do cleanup before exit directly
	os.Exit(1)
}

// FIXME: source line is in correct because we call Fatal in Fatalf
func (l *Logger) Fatalf(format string, args ...interface{}) {
	l.Fatal(fmt.Sprintf(format, args...))
}