NetApp: Report shared blocks exhaustion

The maximum amount of shared (deduplicated, compressed) data on a Data ONTAP FlexVol (i.e. a Cinder pool) is 640TB. The only thing more surprising about that number is that we have customers hitting it. The symptom is that operations such as cloning Cinder volumes fail because no more blocks may be shared. The fix is to report the level of consumption to the scheduler for optional incorporation into the filter & goodness functions, so that pools nearing the shared block limit may be shielded from further provisioning requests.