DAOS-18552 pool: Fix a PS start-stop race (#17564)

[liw]

The following race happened during a pool create operation, triggered by abnormally slow VMs:

ds_rsvc_start
  start
    pool_svc_alloc_cb
      ds_pool_lookup: OK
....VM slowness causes start timeout, which triggers stop....
                          ds_pool_stop
                            pool->sp_stopping = 1
                            ds_pool_svc_stop: none
  insert
                            wait for ds_pool references: hang

This patch is a quick fix that prevents ds_rsvc_start from inserting a PS to the hash table if the ds_pool is stopping, so that ds_pool_stop won't hang. Manual testing shows that such a pool create operation will now retry and succeed transparently.

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 'rebuild/container_rf.py:RbldContRfTest.test_rebuild_with_container_rf - pool create failed: DER_BUSY(-1012): Device or resource busy'
Status is 'In Review'
Labels: 'ci_master_weekly,weekly_test'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-18552