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

Fix build_prior to handle OSDs that no longer exist in the current map.
Consider them lost.

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

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