Fix gang write late_arrival bug

When a write comes in via dmu_sync_late_arrival, its txg is equal to the
open TXG. If that write gangs, and we have not yet activated the new
gang header feature, and the gang header we pick can store a larger gang
header, we will try to schedule the upgrade for the open TXG + 1. In
debug mode, this causes an assertion to trip. This PR sets the TXG for
activating the feature to be the larger of either the current open TXG
or the syncing TXG + 1.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #17824

Author: pcd1193182

module/zfs/zio.c

@@ -3318,8 +3318,8 @@ zio_write_gang_block(zio_t *pio, metaslab_class_t *mc)
 	} else if (any_failed && candidate > SPA_OLD_GANGBLOCKSIZE &&
 	    spa_feature_is_enabled(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER) &&
 	    !spa_feature_is_active(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER)) {
-		dmu_tx_t *tx =
-		    dmu_tx_create_assigned(spa->spa_dsl_pool, txg + 1);
+		dmu_tx_t *tx = dmu_tx_create_assigned(spa->spa_dsl_pool,
+		    MAX(txg, spa_syncing_txg(spa) + 1));
 		dsl_sync_task_nowait(spa->spa_dsl_pool,
 		    zio_update_feature,
 		    (void *)SPA_FEATURE_DYNAMIC_GANG_HEADER, tx);