MFV r316893:

MFV r316893:
7604 if volblocksize property is the default, it displays as "-" rather than 8K

illumos/illumos-gate@4d86c0eab246bdfddc2dd52410ba808433bd6266
https://github.com/illumos/illumos-gate/commit/4d86c0eab246bdfddc2dd52410ba808433bd6266

https://www.illumos.org/issues/7604
  If a zvol has the default setting for the "volblocksize" property, it is
  8KB. However, it is displayed as "-" (not present), rather than "8K".
  The problem was introduced by:
  commit 25228e830e86924a41243343b1de9daf2d7dd43a
      Author: Matthew Ahrens <mahrens@delphix.com>
      Date:   Thu Nov 17 14:37:24 2016 -0800
  7571 non-present readonly numeric ZFS props do not have default value
  which changed changed get_numeric_property() to indicate that readonly
  default properties are not present. However, zfs_prop_readonly() returns
  TRUE for both readonly and set-once properties (e.g. volblocksize).
  Amusingly, that commit essentially reverted:
  6900484 default volblocksize is no longer being reported correctly
  from November 2009. However, that change was not correct either; the
  correct solution is to only do this check for "truly readonly" (i.e. not
  setonce) properties.
  $ zfs list -t volume -o name,volblocksize
      NAME
  VOLBLOCK
      domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
  archive            -
      domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
  datafile           -
      domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
  external           -
      rpool/dump
  128K
      rpool/swap
  4K
      rpool/swap1
  ===============================================================================

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>