crush: fix straw bucket scaler bugs

There's a problem with the straw scaler values we're calculated for straw
buckets.  Fix two of them!
Is there some reason it's important (here and elsewhere) to preserve the bad version on old tunables?
Granted not doing so does make them map slightly differently from the "real" tunables of that version and I understand wanting to be cautious, but I'm not at all sure we care in this case.