osd/OSDMap: clean up primary selection

osd/OSDMap: clean up primary selection

_pg_to_raw_osds was selecting a primary, but it was generally
getting thrown out since _raw_to_up_osds was picking a *new*
primary after filtering out down and nonexistent osds.  Add a
_pick_primary helper to do it and call that explicitly in the
callers where it can be used by all.

Signed-off-by: Sage Weil <sage@redhat.com>