DAOS-19299 mercury: add patch to fix handling of canceled events

[soumagne]

This would have caused RPCs to remain in flight until timeout was reached

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]

Ticket title is 'DaosCoreTest.test_daos_extend_simple hangs (SIGTERM/timeout) in dfs_extend_fetch_kill()'
Status is 'Open'
https://daosio.atlassian.net/browse/DAOS-19299

[daosbuild3]

Test stage Unit Test with memcheck completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18691/1/testReport/