libnvdimm: re-enable deep flush for pmem devices via fsync()

Re-enable deep flush so that users always have a way to be sure that a
write makes it all the way out to media. Writes from the PMEM driver
always arrive at the NVDIMM since movnt is used to bypass the cache, and
the driver relies on the ADR (Asynchronous DRAM Refresh) mechanism to
flush write buffers on power failure. The Deep Flush mechanism is there
to explicitly write buffers to protect against (rare) ADR failure.  This
change prevents a regression in deep flush behavior so that applications
can continue to depend on fsync() as a mechanism to trigger deep flush
in the filesystem-DAX case.

Fixes: 06e8ccda ("acpi: nfit: Add support for detect platform CPU cache...")
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>