Flush the TLB after idle operation

While a CPU executes the idle thread, it receives no cross-CPU TLB flush
requests. Therefore, the TLB needs to get flushed when leaving the idle
thread to compensate for any missed flush requests.

Fixes ssumpf/foc#9.

kernel/fiasco/src/kern/kernel_thread.cpp

@@ -181,6 +181,7 @@ Kernel_thread::idle_op()
       arch_tickless_idle(cpu);
 
       enable_tlb(cpu);
+      Mem_unit::tlb_flush();
       Rcu::leave_idle(cpu);
       Timer_tick::enable(cpu);
     }