sched: add wait begin/end functions that don't change thread state

these functions can be used when waiting on multiple queues at once, to prevent
the thread state from being changed unexpectedly while initialising a set of wait items.

sched/wait.c

@@ -32,6 +32,26 @@ void thread_wait_end(struct wait_item *waiter, struct waitqueue *q)
 	spin_unlock_irqrestore(&q->wq_lock, flags);
 }
 
+void thread_wait_begin_nosleep(struct wait_item *waiter, struct waitqueue *q)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&q->wq_lock, &flags);
+
+	queue_push_back(&q->wq_waiters, &waiter->w_entry);
+
+	spin_unlock_irqrestore(&q->wq_lock, flags);
+}
+
+void thread_wait_end_nosleep(struct wait_item *waiter, struct waitqueue *q)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&q->wq_lock, &flags);
+
+	queue_delete(&q->wq_waiters, &waiter->w_entry);
+
+	spin_unlock_irqrestore(&q->wq_lock, flags);
+}
+
 void wakeup_queue(struct waitqueue *q)
 {
 	unsigned long flags;