threadshare: Fix a deadlock in used-socket notification

This manifests in a gst-launch-1.0 pipeline using ts-udpsrc, since
notification of used-socket results in the property being read by the
application, and the settings lock causes a deadlock.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1408>

generic/threadshare/src/udpsrc/imp.rs

@@ -375,6 +375,8 @@ impl TaskImpl for UdpSrcTask {
                 )
             })?;
 
+            drop(settings);
+
             self.socket = Some(
                 Socket::try_new(
                     self.element.clone().upcast(),