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 <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Reviewed-by: Paul Dagnelie <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #17395

module/zfs/spa.c

@@ -1683,6 +1683,7 @@ spa_activate(spa_t *spa, spa_mode_t mode)
 	ASSERT(spa->spa_state == POOL_STATE_UNINITIALIZED);
 
 	spa->spa_state = POOL_STATE_ACTIVE;
+	spa->spa_final_txg = UINT64_MAX;
 	spa->spa_mode = mode;
 	spa->spa_read_spacemaps = spa_mode_readable_spacemaps;
 
@@ -2085,6 +2086,11 @@ spa_unload(spa_t *spa)
 			vdev_rebuild_stop_all(spa);
 			l2arc_spa_rebuild_stop(spa);
 		}
+
+		spa_config_enter(spa, SCL_ALL, FTAG, RW_WRITER);
+		spa->spa_final_txg = spa_last_synced_txg(spa) +
+		    TXG_DEFER_SIZE + 1;
+		spa_config_exit(spa, SCL_ALL, FTAG);
 	}
 
 	/*