dax: stop requiring a live device for dax_flush()

Now that dax_flush() is no longer a driver callback (commit c3ca015f
"dax: remove the pmem_dax_ops->flush abstraction"), stop requiring the
dax_read_lock() to be held and the device to be alive.  This is in
preparation for switching filesystem-dax to store pfns instead of
sectors in the radix.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>