libnvdimm, pmem: Complete REQ_FLUSH => REQ_PREFLUSH

Complete the move from REQ_FLUSH to REQ_PREFLUSH that apparently started
way back in v4.8.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>