libnvdimm, pmem: use REQ_FUA, REQ_FLUSH for nvdimm_flush()

Given that nvdimm_flush() has higher overhead than wmb_pmem() (pointer
chasing through nd_region), and that we otherwise assume a platform has
ADR capability when flush hints are not present, move nvdimm_flush() to
REQ_FLUSH context.

Note that we still arrange for nvdimm_flush() to be called even in the
ADR case. We need at least once wmb() fence to push buffered writes in
the cpu out to the ADR protected domain.

Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>