libnvdimm, pmem: use nvdimm_flush() for namespace I/O writes

nsio_rw_bytes() is used to write info block metadata to the namespace,
so it should trigger a flush after every write.  Replace wmb_pmem() with
nvdimm_flush() in this path.

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