lavf/mux: pass options to nested structs of priv data

This commit allows to benefit from implementing child_next
callback for muxers' AVClasses.
Without that, options cannot be set in nested structs.
Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>

libavformat/mux.c

@@ -334,7 +334,7 @@ static int init_muxer(AVFormatContext *s, AVDictionary **options)
         if (of->priv_class) {
             *(const AVClass **)s->priv_data = of->priv_class;
             av_opt_set_defaults(s->priv_data);
-            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
+            if ((ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)
                 goto fail;
         }
     }