rbd: fix error paths in rbd_img_request_fill()

Doing rbd_obj_request_put() in rbd_img_request_fill() error paths is
not only insufficient, but also triggers an rbd_assert() in
rbd_obj_request_destroy():

    Assertion failure in rbd_obj_request_destroy() at line 1867:

    rbd_assert(obj_request->img_request == NULL);

rbd_img_obj_request_add() adds obj_requests to the img_request, the
opposite is rbd_img_obj_request_del().  Use it.

Fixes: http://tracker.ceph.com/issues/7327

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -2252,7 +2252,7 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 
 out_unwind:
 	for_each_obj_request_safe(img_request, obj_request, next_obj_request)
-		rbd_obj_request_put(obj_request);
+		rbd_img_obj_request_del(img_request, obj_request);
 
 	return -ENOMEM;
 }