DAOS-19636 object: read layout under lock in obj_ptr2shards

[mchaarawi]

obj_ptr2shards() read cob_shards_nr, cob_grp_size and cob_grp_nr without cob_lock. A concurrent obj_layout_refresh() (e.g. right after reintegration) frees and rebuilds the layout under the write lock, so a stat/punch/sync could see cob_shards_nr == 0 and trip:

Assertion '*grp_nr == obj->cob_grp_nr' failed: Unmatched grp nr: 0/16

Read the layout under cob_lock, check it matches the request map version, and return -DER_STALE instead of asserting, as obj_dkey2grpidx() does.

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 'Object client may use inconsistent layout data during replica retry'
Status is 'Reopened'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-19636