[kernel] Add clear() method to wait_queue

Allow objects to clear out the wait_queue earlier than waiting for the
destructor by moving that functionality into wait_queue::clear().

src/kernel/wait_queue.h

@@ -37,6 +37,9 @@ public:
     /// Get the spinlock to lock this queue
     util::spinlock & get_lock() { return m_lock; }
 
+    /// Wake and clear out all threads.
+    void clear();
+
 private:
     /// Get rid of any exited threads that are next
     /// in the queue. Caller must hold the queue lock.