cthulhu: fix parent_bucket_by_node_id return to be serializable
cthulhu: fix parent_bucket_by_node_id return to be serializable

This info gets sent across in a messagepack and hence needs to be
able to be serialized which a defaultdict cannot.

Fixes: #10446
Signed-off-by: Gregory Meno <4c6f7d69a238d85f14b1affaec7960ea26aa1c6d@redhat.com>

r0h4n/ceph-integration