crush: make set_chooseleaf_tries work with firstn chooseleaf, too

I previous thought we had screwed this up because it wasn't checking
for collide (only reject), but I was wrong (you can't have a collide
when doing the recursive chooseleaf call).  Which means the descend_once
stuff really just means a different default for chooseleaf_tries, and
the result is pretty simple and clean!  Yay.