Possibility to physically reserve space without writing leaf blocks

@@ -49,10 +49,11 @@
         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)