make starpu_data_unpartition acquire the child pieces, otherwise we may not have collapsed the reduction

src/datawizard/filters.c

@@ -315,7 +315,8 @@ void starpu_data_unpartition(starpu_data_handle_t root_handle, unsigned gatherin
 		}
 
 		int ret;
-		ret = _starpu_fetch_data_on_node(child_handle, &child_handle->per_node[gathering_node], STARPU_R, 0, 0, NULL, NULL);
+		/* Acquire the child data. This will may trigger collapsing a reduction */
+		ret = starpu_data_acquire_on_node(child_handle, gathering_node, STARPU_RW);
 		/* for now we pretend that the RAM is almost unlimited and that gathering
 		 * data should be possible from the node that does the unpartionning ... we
 		 * don't want to have the programming deal with memory shortage at that time,