Set spa_final_txg in spa_unload()

[amotin]

I've noticed that after some dedup tests system reboot ends up in assertion about ms_defer tree not empty. It seems to be caused by DDT flushing still freeing some blocks while ZFS trying to reach a final steady state due to spa_final_txg, while being set by spa_export_common() on pool export, is not set when spa_unload() is called by spa_evict_all() on system reboot/shutdown. Setting spa_final_txg in spa_unload() fixes this issue.

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.

[robn]

Yeah, nice. Our chat history suggests we've looked sideways at this a couple of times in the last couple of years, but apparently it was never quite the thing, so we didn't stop. I'm glad I recognised it at least :)

[behlendorf]

Good find.

[pcd1193182]

Just the one question. It's certainly not a big deal, though, I would be happy with the PR in its current state.