rbd: factor out get_parent_info()

In preparation for the new parent_get and parent_overlap_get class
methods, factor out the fetching and decoding of parent data.

As a side effect, we now decode all four fields in the "no parent"
case.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>