DAOS-18531 vos: iterator debugging

[NiuYawei]

Debugging iterator.

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 'mdonssd phase 2: Assertion 'set_entry->se_create_idx != NULL' failed'
Status is 'In Progress'
Labels: 'md_on_ssd2,test_2.8'
https://daosio.atlassian.net/browse/DAOS-18531