DAOS-16091 umem: reset rc properly (#14621)

[NiuYawei]

in umem_cache_checkpoint(), reset the rc to 0 once so_flush_prep() failed for a chkpt_data, otherwise, the rc could be mistakenly used for so_flush_post() for a prior prepared chkpt_data.

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 'DER_AGAIN considered as a fatal error during checkpointing'
Status is 'In Progress'
Job should run at elevated priority (1)
Errors are Unknown component
https://daosio.atlassian.net/browse/DAOS-16091

[daosbuild1]

Test stage Build RPM on EL 8 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/1/execution/node/295/log

[daosbuild1]

Test stage Build RPM on EL 9 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/1/execution/node/402/log

[daosbuild1]

Test stage Build RPM on Leap 15.5 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/1/execution/node/292/log

[daosbuild1]

Test stage Build DEB on Ubuntu 20.04 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/1/execution/node/374/log

[daosbuild1]

Test stage NLT on EL 8.8 completed with status UNSTABLE. https://build.hpdd.intel.com/job/daos-stack/job/daos//view/change-requests/job/PR-14630/1/testReport/

[daosbuild1]

Test stage Build RPM on EL 8 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/2/execution/node/295/log

[daosbuild1]

Test stage Build RPM on EL 9 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/2/execution/node/376/log

[daosbuild1]

Test stage Build RPM on Leap 15.5 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/2/execution/node/290/log

[daosbuild1]

Test stage Build DEB on Ubuntu 20.04 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14630/2/execution/node/362/log