libnvdimm for 4.18

* DAX broke a fundamental assumption of truncate of file mapped pages.
  The truncate path assumed that it is safe to disconnect a pinned page
  from a file and let the filesystem reclaim the physical block. With DAX
  the page is equivalent to the filesystem block. Introduce
  dax_layout_busy_page() to enable filesystems to wait for pinned DAX
  pages to be released. Without this wait a filesystem could allocate
  blocks under active device-DMA to a new file.

* DAX arranges for the block layer to be bypassed and uses
  dax_direct_access() + copy_to_iter() to satisfy read(2) calls.
  However, the memcpy_mcsafe() facility is available through the pmem
  block driver. In order to safely handle media errors, via the DAX
  block-layer bypass, introduce copy_to_iter_mcsafe().

* Fix cache management policy relative to the ACPI NFIT Platform
  Capabilities Structure to properly elide cache flushes when they are not
  necessary. The table indicates whether CPU caches are power-fail
  protected. Clarify that a deep flush is always performed on
  REQ_{FUA,PREFLUSH} requests.
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