rbd: move rbd_dev_refresh() definition

commit 0b035401 upstream.

Move rbd_dev_refresh() definition further down to avoid having to
move struct parent_image_info definition in the next commit.  This
spares some forward declarations too.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
[idryomov@gmail.com: backport to 5.10-6.1: context]
Signed-off-by: Sasha Levin <sashal@kernel.org>