ReplicatedPG::recover_replicas: do not assume that missing objects are unfound

Fixes: #7618
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1381 from ceph/wip-7618

ReplicatedPG::recover_replicas: do not assume that missing objects are u...

Reviewed-by: Sage Weil <sage@inktank.com>