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>
(cherry picked from commit d76936b92300be5cc403fd5a36616a2424c7877d)

Conflicts:
	src/osd/OSD.cc
OSD::calc_priors_during: handle CRUSH_ITEM_NONE correctly

Fixes: #8507
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 0bd6f6797c69af9aff851033c57c42121671c684)

Conflicts:
	src/osd/OSD.cc