nic_router: avoid marginal timeout updates

The link dissolve timeout is updated for every packet, which leads to
trigger_once() RPCs that only marginally change the scheduled timeout but
significantly slow down the packet throughput.

genodelabs/genode#4555

repos/base/include/timer/timeout.h

@@ -123,6 +123,8 @@ class Genode::Timeout : private Noncopyable,
 		void discard();
 
 		bool scheduled();
+
+		Microseconds deadline() const { return _deadline; }
 };