DAOS-18984 dfs: dfs_obj_set_oclass should update the dir open handle

[mchaarawi]

• dfs_obj_set_oclass() changes the on-disk object class for children objects created in the directory, however misses updating that on the open handle of the directory.
• Fix some tests to account for small systems.
• EC oclass will not be chosen if there are not enough Fault Domains, so the dir oclass will actually use what we set on the parent dir instead of the container default.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'dfs_obj_set_oclass does not modify the oclass on the open handle'
Status is 'Open'
Labels: 'triaged'
https://daosio.atlassian.net/browse/DAOS-18984