crimson/osd: fix boot crash when a peer OSD was purged across an osdmap batch

crimson/osd: fix boot crash when a peer OSD was purged across an osdmap batch

OSD::committed_osd_maps() captures old_map, then for each peer in
old_map->get_all_osds() that transitioned up->down it marks the peer's
cluster address down. The address was fetched from the *new* osdmap via
osdmap->get_cluster_addrs(osd_id), but osd_id comes from old_map. If that
peer was purged (fully removed) in the new map, get_cluster_addrs hits
ceph_assert(exists()) and the OSD aborts on boot.

This makes it impossible to decommission a crimson cluster: purging any
OSD leaves surviving crimson OSDs unable to reboot once they replay an
osdmap epoch range spanning that purge, since they abort on this assert
during boot.

Look the address up in old_map instead: it is guaranteed to contain
osd_id (that is where the id came from) and holds its last-known cluster
address, which is exactly what we want to mark down.

Signed-off-by: Vitaly Goot <vgoot@akamai.com>