lavfi/lensfun: Use ff_filter_get_nb_threads() get number of threads

ff_filter_get_nb_threads() respect AVFilterContext.nb_threads and
graph->nb_threads both, in most case, we perfer this API than using
ctx->graph->nb_threads directly.
Reviewed-by: Steven Liu <lq@onvideo.cn>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/vf_lensfun.c

@@ -465,7 +465,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
                                vignetting_filter_slice,
                                &vignetting_thread_data,
                                NULL,
-                               FFMIN(outlink->h, ctx->graph->nb_threads));
+                               FFMIN(outlink->h, ff_filter_get_nb_threads(ctx)));
     }
 
     if (lensfun->mode & (GEOMETRY_DISTORTION | SUBPIXEL_DISTORTION)) {
@@ -493,7 +493,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
                                distortion_correction_filter_slice,
                                &distortion_correction_thread_data,
                                NULL,
-                               FFMIN(outlink->h, ctx->graph->nb_threads));
+                               FFMIN(outlink->h, ff_filter_get_nb_threads(ctx)));
 
         av_frame_free(&in);
         return ff_filter_frame(outlink, out);