Removed an overly restrictive BUG_ON assert from scheduler code.

sched_resume_async() used to forbit current tasks to wake up themselves
since it seems tasks can never be runnable to wake themselves up. However
there's a special case in the scheduler where a task that is about to sleep
may notice it has a pending event and wake itself up asynchronously. Since
all sleeping preparation has already been done and scheduler code is a safe
zone, it is safe to undo it all and resume about-to-sleep task in the scheduler.

We may want to put a BKPT in the pager's suspend routine if it waits for the
sleeping task to resume itself, to see if such a wait is successful. It rarely happens.

src/generic/scheduler.c

@@ -201,11 +201,11 @@ void sched_resume_sync(struct ktcb *task)
 /*
  * Asynchronously resumes a task.
  * The task will run in the future, but at
- * the scheduler's discretion.
+ * the scheduler's discretion. It is possible that current
+ * task wakes itself up via this function in the scheduler().
  */
 void sched_resume_async(struct ktcb *task)
 {
-	BUG_ON(task == current);
 	task->state = TASK_RUNNABLE;
 	sched_rq_add_task(task, rq_runnable, RQ_ADD_FRONT);
 }