DAOS-18870 DFS: Reserve a new DFS OID for GIT.

[sherintg]

Reserve a new DFS OID for the Global Index Table (GIT) used by hardlink support. Additionally bump the DFS_SB_VERSION so that older clients do not operate on containers with GIT enabled.

This change:

• bumps DFS_SB_VERSION from 2 to 3
• adds GIT_HI and DFS_SB_VERSION_HLS constants
• reserves roots.cr_oids[2] for the GIT object at container create time
• extends open_sb() to return sb_ver to mount callers
• gates loading of the new roots slot on sb_ver >= DFS_SB_VERSION_HLS
• stores and restores git_oid in DFS global/local handle conversion
• opens and closes the GIT object handle during mount and unmount
• avoids runtime OID hi collisions with the reserved GIT hi value

This is plumbing only for the reserved object identity and version gating.

Manual Testing:

1. Create two containers one say cont1 with client library without fix and cont2 with client library with the fix.
2. dfuse mount cont1 using the old and the new dfuse. The mount should succeed.
3. dfuse mount cont2 using the old and the new dfuse. With the new dfuse it succeeds and with the old dfuse it fails and the below message is printed in the log:
   2026/04/22 03:36:00.271383 delphi-018 DAOS[634484/634484/0] dfs ERR src/client/dfs/common.c:906 open_sb() Incompatible SB version: 22 (Invalid argument)
   2026/04/22 03:36:00.271418 delphi-018 DAOS[634484/634484/0] dfuse ERR src/client/dfuse/dfuse_core.c:900 dfuse_cont_open(0x5641f23cb290) dfs mount() failed: 22 (Invalid argument)

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 'Reserve a new DFS OID for the Global Index Table (GIT) used by hardlink support and bump SB_VERSION'
Status is 'Open'
Errors are Component should be lower-case
https://daosio.atlassian.net/browse/DAOS-18870

[mchaarawi]

looks good to me, just a couple of nits

[daosbuild3]

Test stage Functional on EL 9 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18084/2/testReport/

[daosbuild3]

Test stage Functional on EL 9 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18084/3/execution/node/997/log

[daosbuild3]

Test stage Functional on EL 9 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18084/4/execution/node/998/log

[knard38]

If I am correct the OID collision issue should be fixed before landing.

[knard38]

LGTM

[daosbuild3]

Test stage Functional on EL 9 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18084/5/testReport/

[sherintg]

@daos-stack/daos-gatekeeper The clang format failure does not appear to be correct and also the functional test for EL 9 appears to be due to missing rpm. Can you please approve and merge the pull request.

[daltonbohning]

Removing gatekeeper for now since CI is still running

[daosbuild3]

Test stage Functional on EL 9 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18084/7/execution/node/980/log