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!
I think the current behavior is least surprising, even if it does mean that someone who sets tunables to bobtail on a fresh cluster gets bad straw behavior.  The alternatives are:

- setting *any* tunable preset switches to the new behavior
- the 'default' and 'optimal' settings pick set straw behavior, but the named profiles (bobtail, firefly, hammer, etc) do not touch it.