BUG/MINOR: hlua_fcn/queue: use atomic load to fetch queue size

In hlua_queue_size(), queue size is loaded as a regular int, but the
queue might be shared by multiple threads that could perform some
atomic pushing or popping attempts in parallel, so we better use an
atomic load operation to guarantee consistent readings.

This could be backported in 2.8.

src/hlua_fcn.c

@@ -542,7 +542,7 @@ static int hlua_queue_size(lua_State *L)
 	struct hlua_queue *queue = hlua_check_queue(L, 1);
 
 	BUG_ON(!queue);
-	lua_pushinteger(L, queue->size);
+	lua_pushinteger(L, HA_ATOMIC_LOAD(&queue->size));
 
 	return 1;
 }