3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument is zero

*** 342,352 ****
          if (0 == zfeature_lookup_guid(feature.fi_guid, NULL))
                  fatal("'%s' is a real feature, will not enable");
          if (0 == zap_contains(mos, spa->spa_feat_desc_obj, feature.fi_guid))
                  fatal("feature already enabled: %s", feature.fi_guid);
  
!         VERIFY3U(0, ==, dsl_sync_task_do(spa->spa_dsl_pool, NULL,
              feature_enable_sync, spa, &feature, 5));
  
          spa_close(spa, FTAG);
  
          free(desc);
--- 342,352 ----
          if (0 == zfeature_lookup_guid(feature.fi_guid, NULL))
                  fatal("'%s' is a real feature, will not enable");
          if (0 == zap_contains(mos, spa->spa_feat_desc_obj, feature.fi_guid))
                  fatal("feature already enabled: %s", feature.fi_guid);
  
!         VERIFY0(dsl_sync_task_do(spa->spa_dsl_pool, NULL,
              feature_enable_sync, spa, &feature, 5));
  
          spa_close(spa, FTAG);
  
          free(desc);
*** 440,450 ****
          }
  
          if (decr && !spa_feature_is_active(spa, &feature))
                  fatal("feature refcount already 0: %s", feature.fi_guid);
  
!         VERIFY3U(0, ==, dsl_sync_task_do(spa->spa_dsl_pool, NULL,
              decr ? feature_decr_sync : feature_incr_sync, spa, &feature, 5));
  
          spa_close(spa, FTAG);
  }
  
--- 440,450 ----
          }
  
          if (decr && !spa_feature_is_active(spa, &feature))
                  fatal("feature refcount already 0: %s", feature.fi_guid);
  
!         VERIFY0(dsl_sync_task_do(spa->spa_dsl_pool, NULL,
              decr ? feature_decr_sync : feature_incr_sync, spa, &feature, 5));
  
          spa_close(spa, FTAG);
  }