Address a race condition that was introduced in r238212. Unless the page

queues lock is acquired before the page lock is released, there is no
guarantee that the page will still be in that same page queue when
vm_page_requeue() is called.

Reported by:		pho
In collaboration with:	kib
MFC after:	3 days

sys/vm/vm_pageout.c

@@ -1125,9 +1125,9 @@ vm_pageout_scan(int pass)
 			 * Those objects are in a "rundown" state.
 			 */
 			if (!swap_pageouts_ok || (object->flags & OBJ_DEAD)) {
+				vm_page_lock_queues();
 				vm_page_unlock(m);
 				VM_OBJECT_UNLOCK(object);
-				vm_page_lock_queues();
 				queues_locked = TRUE;
 				vm_page_requeue(m);
 				goto relock_queues;