Use received frames for period calculation

out.frames was clock.duration at some point but it changed, and this
didn't

src/pipewire-audio.c

@@ -206,8 +206,7 @@ static void on_process_cb(void *data)
 		  * (https://gitlab.freedesktop.org/pipewire/pipewire/-/blob/0.3.52/pipewire-jack/src/pipewire-jack.c#L5639)
 		  * which is used in the linux-jack plugin to correctly set the timestamp
 		  * (https://github.com/obsproject/obs-studio/blob/27.2.4/plugins/linux-jack/jack-wrapper.c#L87) */
-		double period_nsecs =
-			s->pos->clock.duration * (double)SPA_NSEC_PER_SEC / (s->info.sample_rate * s->pos->clock.rate_diff);
+		double period_nsecs = out.frames * (double)SPA_NSEC_PER_SEC / (s->info.sample_rate * s->pos->clock.rate_diff);
 
 		out.timestamp = now - (uint64_t)period_nsecs;
 	} else {