osd: handle osds that no longer exist in prior_set_affected
osd: handle osds that no longer exist in prior_set_affected

Consider no-longer-existent OSDs lost.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

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