DAOS-16211 vos: Avoid race condition with discard (#15370)

[jolivier23]

There is a possible race between aggregation deleting the object tree and discard working on the same
object tree. Add a check to avoid this race

Before requesting gatekeeper:

• Two review approvals and any prior change requests have been resolved.
• Testing is complete and all tests passed or there is a reason documented in the PR why it should be force landed and forced-landing tag is set.
• Features: (or Test-tag*) commit pragma was used or there is a reason documented that there are no appropriate tags for this PR.
• Commit messages follows the guidelines outlined here.
• Any tests skipped by the ticket being addressed have been run and passed in the PR.

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• Checkpatch issues are resolved. Pay particular attention to ones that will show up on future PRs.
• All builds have passed. Check non-required builds for any new compiler warnings.
• Sufficient testing is done. Check feature pragmas and test tags and that tests skipped for the ticket are run and now pass with the changes.
• If applicable, the PR has addressed any potential version compatibility issues.
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• Extra checks if forced landing is requested
  • Review comments are sufficiently resolved, particularly by prior reviewers that requested changes.
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  • Quick-build or Quick-functional is not used.
• Fix the commit message upon landing. Check the standard here. Edit it to create a single commit. If necessary, ask submitter for a new summary.

[github-actions]

Ticket title is 'Assertion in VOS aggregation (close_merge_window() Assertion '(mw->mw_rmv_cnt == 0)' )'
Status is 'In Progress'
Labels: 'daos_ecb_issue,daos_ecb_scale,soak,triaged,request_for_2.6.2'
https://daosio.atlassian.net/browse/DAOS-16211

[daosbuild1]

Test stage Functional Hardware Medium completed with status FAILURE. https://build.hpdd.intel.com/job/daos-stack/job/daos/job/PR-15432/4/display/redirect

[daosbuild1]

Test stage Functional Hardware Medium Verbs Provider completed with status FAILURE. https://build.hpdd.intel.com/job/daos-stack/job/daos/job/PR-15432/4/display/redirect