avfilter/af_loudnorm: fix incorrect gain when audio is shorter than 3s

The input data is multiplied by `s->offset` to get normalized output.
`s->target_tp` and `true_peak` is not in dB,
so `s->offset` should be calculated by division instead of subtraction.

Signed-off-by: Rui Zhu <real.zhurui@gmail.com>

libavfilter/af_loudnorm.c

@@ -455,7 +455,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
 
         offset    = pow(10., (s->target_i - global) / 20.);
         offset_tp = true_peak * offset;
-        s->offset = offset_tp < s->target_tp ? offset : s->target_tp - true_peak;
+        s->offset = offset_tp < s->target_tp ? offset : s->target_tp / true_peak;
         s->frame_type = LINEAR_MODE;
     }