- Introduce 'zero_page_range' as a dax operation. This facilitates
  filesystem-dax operation without a block-device.

- Advertise a persistence-domain for of_pmem and papr_scm. The
  persistence domain indicates where cpu-store cycles need to reach in
  the platform-memory subsystem before the platform will consider them
  power-fail protected.

- Fixup some flexible-array declarations.