rbd: img_data requests don't own their page array

Move the check into rbd_obj_request_destroy() to avoid use-after-free
on errors in rbd_img_request_fill(..., OBJ_REQUEST_PAGES, ...), where
pages, owned by the caller, gets freed in rbd_img_request_fill().
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>

drivers/block/rbd.c

@@ -2147,7 +2147,9 @@ static void rbd_obj_request_destroy(struct kref *kref)
 			bio_chain_put(obj_request->bio_list);
 		break;
 	case OBJ_REQUEST_PAGES:
-		if (obj_request->pages)
+		/* img_data requests don't own their page array */
+		if (obj_request->pages &&
+		    !obj_request_img_data_test(obj_request))
 			ceph_release_page_vector(obj_request->pages,
 						obj_request->page_count);
 		break;
@@ -2368,13 +2370,6 @@ static bool rbd_img_obj_end_request(struct rbd_obj_request *obj_request)
 		xferred = obj_request->length;
 	}
 
-	/* Image object requests don't own their page array */
-
-	if (obj_request->type == OBJ_REQUEST_PAGES) {
-		obj_request->pages = NULL;
-		obj_request->page_count = 0;
-	}
-
 	if (img_request_child_test(img_request)) {
 		rbd_assert(img_request->obj_request != NULL);
 		more = obj_request->which < img_request->obj_request_count - 1;