#18172: Detect and heal orphaned DTL entries on pool import

[nmarasoiu]

#18172

When a system crashes during vdev detach operations, the on-disk state can become inconsistent: the vdev tree shows the device as a "hole" or "missing", but DTL entries from the detached device remain on disk.

On pool import, these orphaned DTL entries can trigger a phantom resilver that scans the entire pool with no valid target device, causing severe I/O load and system instability.

This patch adds:

• vdev_dtl_check_orphaned(): detect orphaned DTL on hole/missing vdevs
• New import flag ZFS_IMPORT_HEAL_ORPHANED_DTL (0x100)
• CLI option: zpool import -o heal_orphaned_dtl=on poolname
• Clear warning messages to inform the administrator

By default, import fails with a helpful error message when orphaned DTL entries are detected, directing the user to re-import with the healing option. This ensures administrators are explicitly aware of and consent to the recovery action.

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

Would we have transactions to handle atomic operations... ;)