Illumos 6292 - exporting a pool while an async destroy

6292 exporting a pool while an async destroy is running can leave
entries in the deferred tree
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Fabian Keil <fk@fabiankeil.de>
Approved by: Gordon Ross <gordon.ross@nexenta.com>

References:
  https://www.illumos.org/issues/6292
  illumos/illumos-gate@a443cc8

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

Author: grwilson

module/zfs/dsl_scan.c

@@ -1470,11 +1470,24 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 		dsl_scan_setup_sync(&func, tx);
 	}
 
+	/*
+	 * Only process scans in sync pass 1.
+	 */
+	if (spa_sync_pass(dp->dp_spa) > 1)
+		return;
+
+	/*
+	 * If the spa is shutting down, then stop scanning. This will
+	 * ensure that the scan does not dirty any new data during the
+	 * shutdown phase.
+	 */
+	if (spa_shutting_down(spa))
+		return;
+
 	/*
 	 * If the scan is inactive due to a stalled async destroy, try again.
 	 */
-	if ((!scn->scn_async_stalled && !dsl_scan_active(scn)) ||
-	    spa_sync_pass(dp->dp_spa) > 1)
+	if (!scn->scn_async_stalled && !dsl_scan_active(scn))
 		return;
 
 	scn->scn_visited_this_txg = 0;