perf(ext4): prevent lmbench ext4_create_delete_files_0k_ops timeout by indexing the orphan chain

[mistcoversmyeyes]

Replaces #2346.

Fixes #2327. Related: #2286.

A large final-unlink backlog repeatedly validates the entire ext4 legacy orphan chain and walks it again to find a reclaim predecessor. This makes create/delete batches increasingly expensive. The default lat_fs size sweep exceeded its original 120-second deadline; a stopped-state sample during deletion found validate_chain called by legacy_orphan_contains / unlink_inode.

Keep a membership/predecessor index for the validated logical chain. Initial use and any aborted index update still validate the complete chain, while normal add/remove transitions update the index under the existing metadata mutation gate. Removal rereads the target and predecessor and verifies allocation, checksums, inode generation and links. Transaction drop/abort invalidates provisional changes; successful logical publication retains them, and an older batch checkpoint cannot revive an invalidated index. Replay invalidates the index. No on-disk format or journal credit budget changes.

• Baseline original wrapper exceeded 120 seconds. In a fresh fixed guest with ENOUGH, TIMING_O and LOOP_O explicitly unset, the original ext4_create_delete_files_0k_ops.sh completed all default 0/1/4/10 KiB stages with exit status 0 within the same deadline. No size selection, calibration parameter or deadline was changed; this is a completion check, not a controlled throughput comparison.
• make kernel, rustfmt and git diff --check passed.
• 206 another_ext4 unit tests passed, including index transitions, abort/drop, successful direct/journal/batch publication, and journal failure boundaries.
• Real-image namespace_transactions (including both persistence models), batched_delalloc, and batched_fragmented_reclaim regressions passed with their filesystem consistency checks.
• Reclaim, linked-tail final unlink and linked-tail rename recovery fault matrices passed for writeback and writethrough persistence.