fallbacksrc: Flush source before restarting on errors

It might still be blocked downstream for a while, e.g. in the clocksync.

Flushing does not cause any problems as fallbackswitch is not going to
forward it and will only unblock everything up to there.

utils/fallbackswitch/src/fallbacksrc/imp.rs

@@ -2108,6 +2108,9 @@ impl FallbackSrc {
             }
             drop(state_guard);
 
+            gst_debug!(CAT, obj: element, "Flushing source");
+            let _ = source.send_event(gst::event::FlushStart::builder().build());
+
             gst_debug!(CAT, obj: element, "Shutting down source");
             let _ = source.set_state(gst::State::Null);