Using a new object for snapmapper is tricky. The only existing object like this is the pg meta object, which is special-cased in a variety of places through the osd. It also happens to have the nice property that it's not expressible through a normal rados operation -- rados objects are not allowed to have empty names.

This snapmapper object, on the other hand, could actually be the target of a user read or write (though it's unlikely to happen by accident). In order to do this correctly, the existing special cases for the pgmeta object would need to be generalized and the normal user IO path would need to disallow mutations (and maybe reads since these objects wouldn't follow normal rules).

Instead, let's just stick these keys into the pgmeta object and save ourselves a lot of work.

Fixes: https://tracker.ceph.com/issues/64975

https://pulpito.ceph.com/sjust-2024-03-21_02:35:36-crimson-rados-wip-sjust-crimson-testing-2024-03-20-distro-default-smithi/

Failures unrelated, caused by existing known bugs.