osd: make prior_set_affected() slightly smarter
osd: make prior_set_affected() slightly smarter

Only return true if an osd goes down that we didn't already know was
down (prior_set may contain down osds if the PG is marked DOWN).

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph