Possibility to physically reserve space without writing leaf blocks

*** 141,150 ****
--- 141,152 ----
                          override_states_t dr_override_state;
                          uint8_t dr_copies;
                          boolean_t dr_nopwrite;
                  } dl;
          } dt;
+ 
+         boolean_t dr_zero_write;
  } dbuf_dirty_record_t;
  
  typedef struct dmu_buf_impl {
          /*
           * The following members are immutable, with the exception of
*** 273,285 ****
  dmu_buf_impl_t *dbuf_find(struct dnode *dn, uint8_t level, uint64_t blkid);
  
  int dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags);
  void dmu_buf_will_not_fill(dmu_buf_t *db, dmu_tx_t *tx);
  void dmu_buf_will_fill(dmu_buf_t *db, dmu_tx_t *tx);
  void dmu_buf_fill_done(dmu_buf_t *db, dmu_tx_t *tx);
  void dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx);
! dbuf_dirty_record_t *dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
  arc_buf_t *dbuf_loan_arcbuf(dmu_buf_impl_t *db);
  void dmu_buf_write_embedded(dmu_buf_t *dbuf, void *data,
      bp_embedded_type_t etype, enum zio_compress comp,
      int uncompressed_size, int compressed_size, int byteorder, dmu_tx_t *tx);
  
--- 275,289 ----
  dmu_buf_impl_t *dbuf_find(struct dnode *dn, uint8_t level, uint64_t blkid);
  
  int dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags);
  void dmu_buf_will_not_fill(dmu_buf_t *db, dmu_tx_t *tx);
  void dmu_buf_will_fill(dmu_buf_t *db, dmu_tx_t *tx);
+ void dmu_buf_will_zero_fill(dmu_buf_t *db, dmu_tx_t *tx);
  void dmu_buf_fill_done(dmu_buf_t *db, dmu_tx_t *tx);
  void dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx);
! dbuf_dirty_record_t *dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx, boolean_t zero_write);
! dbuf_dirty_record_t *dbuf_zero_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
  arc_buf_t *dbuf_loan_arcbuf(dmu_buf_impl_t *db);
  void dmu_buf_write_embedded(dmu_buf_t *dbuf, void *data,
      bp_embedded_type_t etype, enum zio_compress comp,
      int uncompressed_size, int compressed_size, int byteorder, dmu_tx_t *tx);