DAOS-17589 pool: not changing fseq for exclude during drain

[wangdi1]

If the target is being drained, then exclude should not change its fseq number, otherwise it may get wrong object layout and interfere the drain process.

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[bot]]

Ticket title is 'exclude should not change the fseq number during drain' Status is 'Open' https://daosio.atlassian.net/browse/DAOS-17589