4171 clean up spa_feature_*() interfaces
4172 implement extensible_dataset feature for use by other zpool features
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

*** 603,613 ****
                                  return (err);
                          lasterr = err;
                          continue;
                  }
  
!                 if (doi.doi_type == DMU_OT_DSL_DATASET) {
                          dsl_dataset_t *ds;
                          uint64_t txg = txg_start;
  
                          dsl_pool_config_enter(dp, FTAG);
                          err = dsl_dataset_hold_obj(dp, obj, FTAG, &ds);
--- 603,613 ----
                                  return (err);
                          lasterr = err;
                          continue;
                  }
  
!                 if (doi.doi_bonus_type == DMU_OT_DSL_DATASET) {
                          dsl_dataset_t *ds;
                          uint64_t txg = txg_start;
  
                          dsl_pool_config_enter(dp, FTAG);
                          err = dsl_dataset_hold_obj(dp, obj, FTAG, &ds);