DAOS-16556 client: call fstat() before mmap() to update file status in kernel

[wiliamhuang]

(#15244)

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.

Gatekeeper:

• You are the appropriate gatekeeper to be landing the patch.
• The PR has 2 reviews by people familiar with the code, including appropriate owners.
• Githooks were used. If not, request that user install them and check copyright dates.
• 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.
• Check the target branch. If it is master branch, should the PR go to a feature branch? If it is a release branch, does it have merge approval in the JIRA ticket.
• Extra checks if forced landing is requested
  • Review comments are sufficiently resolved, particularly by prior reviewers that requested changes.
  • No new NLT or valgrind warnings. Check the classic view.
  • 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 'dfuse/daos_build.py:DaosBuild.test_dfuse_daos_build_wt_pil4df - Failure to build over dfuse with il in mode nocache'
Status is 'Awaiting backport'
Labels: '2.6.1rc1,2.6.1rc2,ci_master_weekly,intercept_lib,weekly_test'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-16556

[wiliamhuang]

All tests in CI passed except the NLT issue which is not related to this PR.