rbd: call rbd_dev_mapping_set() from rbd_dev_image_probe()

Snapshot object map will be loaded in rbd_dev_image_probe(), so we need
to know snapshot's size (as opposed to HEAD's size) sooner.

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