Possibility to physically reserve space without writing leaf blocks

*** 49,58 ****
--- 49,59 ----
          SPA_FEATURE_HOLE_BIRTH,
          SPA_FEATURE_EXTENSIBLE_DATASET,
          SPA_FEATURE_EMBEDDED_DATA,
          SPA_FEATURE_BOOKMARKS,
          SPA_FEATURE_FS_SS_LIMIT,
+         SPA_FEATURE_SPACE_RESERVATION,
          SPA_FEATURES
  } spa_feature_t;
  
  #define SPA_FEATURE_DISABLED    (-1ULL)