DAOS-18190 container: Fix uninitialized op_vals

[liw]

The reserved bytes in ds_pool_svc_op_val are not zeroed before being written to the DB. This patch fixes the issue for the 2.8 pool global version.

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 'A container destroy operation should prevent accesses before starting to destroy the data'
Status is 'In Progress'
https://daosio.atlassian.net/browse/DAOS-18190

[liw]

Upgrading a 2.6.4 pool worked:

2026/03/10 05:07:34.176277 brd-226 DAOS[1553764/0/1029] pool DBUG src/pool/srv_pool.c:6158 pool_upgrade_props() d665ab5b: check ds_pool_prop_svc_ops
2026/03/10 05:07:34.176286 brd-226 DAOS[1553764/0/1029] vos  DBUG src/vos/vos_io.c:1462 akey_fetch() akey [8] 'svc_ops uint64:32493245965760115' fetch single epr 3-5e
2026/03/10 05:07:34.176295 brd-226 DAOS[1553764/0/1029] pool INFO src/pool/srv_pool.c:6170 pool_upgrade_props() d665ab5b: destroying and recreating service ops KVS
2026/03/10 05:07:34.176308 brd-226 DAOS[1553764/0/1029] pool DBUG src/pool/srv_pool.c:6183 pool_upgrade_props() d665ab5b: creating service ops KVS

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-17666/2/execution/node/1338/log