Possibility to physically reserve space without writing leaf blocks

@@ -141,10 +141,12 @@
                         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

@@ -273,13 +275,15 @@
 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);
+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);