DAOS-17345 vos: cap reserved space

[NiuYawei]

When there is already lots of space reserved for aggregation/GC (for example, md-on-ssd phase GC will reserve 800k per 16MB), we shouldn't add up the reserved space for GC and anti-fragmentation, instead, we'd use the larger one.

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 'DAOS reserving 20% (in some cases 29%) of SCM on pool creation' Status is 'In Review' Job should run at elevated priority (1) https://daosio.atlassian.net/browse/DAOS-17345

[daosbuild3]

Test stage NLT on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-16371/4/testReport/

[daosbuild3]

Test stage Functional Hardware Medium Verbs Provider MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-16371/7/execution/node/1708/log