Currently dm_dax_flush() is not being called, even if underlying dax
device supports write cache, because DAXDEV_WRITE_CACHE is not being
propagated up to the DM dax device.

If the underlying dax device supports write cache, set
DAXDEV_WRITE_CACHE on the DM dax device.  This will cause dm_dax_flush()
to be called.

Fixes: abebfbe2 ("dm: add ->flush() dax operation support")
Signed-off-by: NVivek Goyal <vgoyal@redhat.com>
Acked-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>