OSD::calc_priors_during: fix confusing for loop bracing (cosmetic)

Confusing lack of braces is confusing.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD::calc_priors_during: handle CRUSH_ITEM_NONE correctly

Fixes: #8507
Signed-off-by: Samuel Just <sam.just@inktank.com>