librbd: retry ENOENT in V2_REFRESH_PARENT as well

librbd: retry ENOENT in V2_REFRESH_PARENT as well

With auto-deletion of trashed snapshots, it is relatively easy to lose
a race to "rbd flatten" as follows:

- when V2_GET_PARENT runs, the image is technically still a clone
- when V2_REFRESH_PARENT runs, the image is fully flattened and the
  snapshot in the parent image is deleted

This results in a spurious ENOENT error, mainly when trying to open the
image (e.g. for "rbd info").  This race condition has always been there
but auto-deletion of trashed snapshots makes it much worse.

Retry ENOENT in V2_REFRESH_PARENT the same way as in V2_GET_SNAPSHOTS.

Fixes: https://tracker.ceph.com/issues/52810
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>