Possibility to physically reserve space without writing leaf blocks

*** 610,619 ****
--- 610,620 ----
  #define DMU_READ_NO_PREFETCH    1 /* don't prefetch */
  int dmu_read(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
          void *buf, uint32_t flags);
  void dmu_write(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
          const void *buf, dmu_tx_t *tx);
+ void dmu_write_zero(objset_t *os, uint64_t object, uint64_t offset, uint64_t size, dmu_tx_t *tx);
  void dmu_prealloc(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
          dmu_tx_t *tx);
  int dmu_read_uio(objset_t *os, uint64_t object, struct uio *uio, uint64_t size);
  int dmu_read_uio_dbuf(dmu_buf_t *zdb, struct uio *uio, uint64_t size);
  int dmu_write_uio(objset_t *os, uint64_t object, struct uio *uio, uint64_t size,