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>

drivers/dax/super.c

@@ -273,9 +273,6 @@ EXPORT_SYMBOL_GPL(dax_copy_from_iter);
 void arch_wb_cache_pmem(void *addr, size_t size);
 void dax_flush(struct dax_device *dax_dev, void *addr, size_t size)
 {
-	if (unlikely(!dax_alive(dax_dev)))
-		return;
-
 	if (unlikely(!test_bit(DAXDEV_WRITE_CACHE, &dax_dev->flags)))
 		return;