librbd: fix crash using clone of flattened image

The crash occurs due to ImageCtx->parent->parent being uninitialized,
since the inital open_parent() -> open_image(parent) ->
ictx_refresh(parent) occurs before ImageCtx->parent->snap_id is set,
so refresh_parent() is not called to open an ImageCtx for the parent
of the parent. This leaves the ImageCtx->parent->parent NULL, but the
rest of ImageCtx->parent updated to point at the correct parent snapshot.

Setting the parent->snap_id earlier has some unintended side effects
currently, so for now just call refresh_parent() during
open_parent(). This is the easily backportable version of the
fix. Further patches can clean up this whole initialization process.

Fixes: #8845
Backport: firefly, dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 2545e80d274b23b6715f4d8b1f4c6b96182996fb)