Fix a case where we did not properly track the ub_seq.

[jeff]

Summary

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.

[jeff]

sys/vm/uma_core.c

562

this was requested by gleb for coverity

[markj]

sys/vm/uma_core.c

985

You can micro-optimize this the same way as zone fetch bucket, to avoid looping over bucket entries when it's not necessary to do so.