Fix waiting in ztest_device_removal()

spa->spa_vdev_removal is created in a sync task that is initiated
via dsl_sync_task_nowait(). Since the task may not run before
spa_vdev_remove() returns, we must wait at least 1 txg to ensure
that the removal struct has been created.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes openzfs#8010

cmd/ztest/ztest.c

@@ -3549,6 +3549,15 @@ ztest_device_removal(ztest_ds_t *zd, uint64_t id)
 		ztest_device_removal_active = B_TRUE;
 		mutex_exit(&ztest_vdev_lock);
 
+		/*
+		 * spa->spa_vdev_removal is created in a sync task that
+		 * is initiated via dsl_sync_task_nowait(). Since the
+		 * task may not run before spa_vdev_remove() returns, we
+		 * must wait at least 1 txg to ensure that the removal
+		 * struct has been created.
+		 */
+		txg_wait_synced(spa_get_dsl(spa), 0);
+
 		while (spa->spa_vdev_removal != NULL)
 			txg_wait_synced(spa_get_dsl(spa), 0);
 	} else {