rbd: fix response length parameter for encoded strings

rbd_dev_image_id() allocates space for length but passes a smaller
value to rbd_obj_method_sync().  rbd_dev_v2_object_prefix() doesn't
allocate space for length.  Fix both to be consistent.

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>