DAOS-18691 pool: make chkpt ULT yield properly

[NiuYawei]

chkpt ULT should always yield to make progress when it failed to acquire enough DMA buffers.

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 'VOS Aggregation Assertion noticed: recx2ext() Assertion 'recx->rx_nr > 0''
Status is 'In Progress'
Labels: 'md_on_ssd,scrubbed_2.8,test_2.8'
https://daosio.atlassian.net/browse/DAOS-18691