uzd_cross did not have a seq set until it was full. If the bucket was reaped in a low memory situation we would not wait for seq to expire or call the dtors. We had a similar problem via uma_zdestroy(). I wanted to place the advance call in cache_bucket_unload() but it just didn't really work out any cleaner.
This now passes invariants with a lot of stress and memory pressure followed by destroying zones.