Kernel: Always return from Thread::wait_on

We need to always return from Thread::wait_on, even when a thread
is being killed. This is necessary so that the kernel call stack
can clean up and release references held by it. Then, right before
transitioning back to user mode, we check if the thread is
supposed to die, and at that point change the thread state to
Dying to prevent further scheduling of this thread.

This addresses some possible resource leaks similar to #3073

Kernel/Process.cpp

@@ -273,10 +273,6 @@ void Process::kill_threads_except_self()
         // At this point, we have no joiner anymore
         thread.m_joiner = nullptr;
         thread.set_should_die();
-
-        if (thread.state() != Thread::State::Dead)
-            thread.set_state(Thread::State::Dying);
-
         return IterationDecision::Continue;
     });