tsnet: shut down the event bus on Close

Updates #15160

Change-Id: I29c8194b4b41e95848e5f160e9970db352588449
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

tsnet/tsnet.go

@@ -435,8 +435,11 @@ func (s *Server) Close() error {
 	for _, ln := range s.listeners {
 		ln.closeLocked()
 	}
-
 	wg.Wait()
+
+	if bus := s.sys.Bus.Get(); bus != nil {
+		bus.Close()
+	}
 	s.closed = true
 	return nil
 }