rbd: move rbd_dev_refresh() definition

commit 0b035401c57021fc6c300272cbb1c5a889d4fe45 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 5.4: drop rbd_is_snap() assert,
 preserve rbd_exists_validate() call]
Signed-off-by: Sasha Levin <sashal@kernel.org>