DAOS-18901 vos: Cap merged extent size

[NiuYawei]

Cap the merged extent size at the defined threshold.

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).

[wangshilong]

Is there a unit test to verify this?

[github-actions]

Ticket title is 'Aurora daos_user:DAOS container aggregation fails intermittently with DER_NOMEM (-1009) during VOS merge window processing.'
Status is 'In Review'
Labels: 'ALCF,scrubbed_2.8'
https://daosio.atlassian.net/browse/DAOS-18901

[NiuYawei]

Is there a unit test to verify this?

no, there isn't unit test for this so far. I think it's not necessary to introduce an unit test for this trivial change?

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18136/1/testReport/

[daosbuild3]

Test stage Functional Hardware Large MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos/job/PR-18136/1/display/redirect

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18136/2/testReport/