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>

*** 248,268 ****
                          goto out;
                  VERIFY0(bpobj_open(&dp->dp_free_bpobj,
                      dp->dp_meta_objset, obj));
          }
  
!         if (spa_feature_is_active(dp->dp_spa,
!             &spa_feature_table[SPA_FEATURE_ASYNC_DESTROY])) {
                  err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                      DMU_POOL_BPTREE_OBJ, sizeof (uint64_t), 1,
                      &dp->dp_bptree_obj);
                  if (err != 0)
                          goto out;
          }
  
!         if (spa_feature_is_active(dp->dp_spa,
!             &spa_feature_table[SPA_FEATURE_EMPTY_BPOBJ])) {
                  err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                      DMU_POOL_EMPTY_BPOBJ, sizeof (uint64_t), 1,
                      &dp->dp_empty_bpobj);
                  if (err != 0)
                          goto out;
--- 248,266 ----
                          goto out;
                  VERIFY0(bpobj_open(&dp->dp_free_bpobj,
                      dp->dp_meta_objset, obj));
          }
  
!         if (spa_feature_is_active(dp->dp_spa, SPA_FEATURE_ASYNC_DESTROY)) {
                  err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                      DMU_POOL_BPTREE_OBJ, sizeof (uint64_t), 1,
                      &dp->dp_bptree_obj);
                  if (err != 0)
                          goto out;
          }
  
!         if (spa_feature_is_active(dp->dp_spa, SPA_FEATURE_EMPTY_BPOBJ)) {
                  err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                      DMU_POOL_EMPTY_BPOBJ, sizeof (uint64_t), 1,
                      &dp->dp_empty_bpobj);
                  if (err != 0)
                          goto out;