osd/ReplicatedPG: tolerate trailing missing clones on cache tiers

I missed this case in eb71924ea27e78d97bd45674ef5e6a7fce30932f.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_rados_api_tier: improve cache tier + scrub test

Create lots of objects and make *some* of them be missing clones but not
all.

Signed-off-by: Sage Weil <sage@inktank.com>