Set spa_final_txg in spa_unload()

I've noticed that after some dedup tests system reboot ends up in
assertion about ms_defer tree not free. 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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17395