Description of problem:
Due to the decentralized approach to RBD image management, it is possible to snapshot an image which is being concurrently flattened.  Since the flatten operation would not have known about the new snapshot, it will detach the image from its parent (if no other snapshots exist).  

This will allow the parent image snapshot to be unprotected and removed since it doesn't know about the new dependent snapshot.

This condition exposes issues that can cause 'rbd ls -l' to crash due to the missing parent and prevents the removal of the corrupt snapshot.

How reproducible:
Race condition

Steps to Reproduce:
1. Clone an image from an origin snapshot
2. Flatten the clone
3. While flattening, snapshot the clone
4. Delete the origin snapshot and image

Actual results:
Orphaned snapshot cannot be removed, 'rbd ls -l' crashes.

Expected results:
Orphaned snapshot should be able to be removed, 'rbd ls -l' shouldn't crash.