DAOS-19645 common: Enhance dav_v2 alloc-from-bucket APIs for multiple…

[sherintg]

… E-buckets

Rework the umem alloc-from-bucket APIs to accept a struct of type umem_bucket_req instead of a single bucket id, so an allocation can describe an ordered set of evictable (E) buckets.

• Add umem_bucket_req_t { ubr_bkt_ids, ubr_bkt_cnt, ubr_bkt_max } and change umem_{alloc,zalloc,reserve,atomic_alloc}_from_bucket() and the backend ops (mo_tx_alloc/mo_reserve/mo_atomic_alloc) to take a umem_bucket_req_t * (NULL == default/non-evictable bucket).

• palloc_reservation_create now allocates by walking the requested evictable buckets in priority order, then the spill-over evictable (SOE) buckets, and finally the non-evictable bucket.

• TEMP FIX: ubr_bkt_max == 0 permits unbounded spill-over to SOE buckets, preserving the legacy single-bucket behavior; the vos callers request ubr_bkt_cnt = 1, ubr_bkt_max = 0. Unbounded SOE spill-over will be removed in a future change.

• Update umem/vos callers and add a unit test (BMEM017a) covering spill-over across two requested buckets.

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 'Enhance dav v2 alloc from bucket APIs to accept the multiple E buckets'
Status is 'Open'
https://daosio.atlassian.net/browse/DAOS-19645

[daosbuild3]

Test stage Unit Test bdev completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-19093/1/testReport/