osd: fix manifest object not to be promoted when references_chunk called

osd: fix manifest object not to be promoted when references_chunk called

When a cls_cas_references_chunk() is called on a chunked metadata object,
it makes the object's chunks be promoted in maybe_handle_manifest_detail().
It happens, for instance, while doing a chunk scrub job.

However, this operation doesn't need to get evicted data.
It only needs metadata information that already exists in metadata object.
To prevent this object promotion, this commit adds an exception handling for this.

Signed-off-by: Sungmin Lee <sung_min.lee@samsung.com>