hammer2 - Synchronize write-in-place feature

hammer2 - Synchronize write-in-place feature

* Disable hole creation when the check mode is disabled on a file.
  Writing zeros maintains any previously assigned block.

* Allows reserving storage for a file (such as an image) by dd'ing
  /dev/zero into it, as long as check mode has also been disabled
  on that file.  No data CRCs or hashes will be computed or checked
  for the file.

* Note that this should still work properly with snapshots.  A snapshot
  will force block reallocation on the master for any writes that cross
  the snapshot boundary.