librbd: open parent image when opening a child

librbd: open parent image when opening a child

Extract a helper out of get_parent_info. The parent may become unset
while the child is open, so detect changes in it during ictx_refresh().
Don't watch the parent image, since we only care about the read-only
snapshot the child references, which cannot change.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>