fs/dax: Clarify nr_pages to dax_direct_access()

dax_direct_access() takes a number of pages.  PHYS_PFN(PAGE_SIZE) is a
very round about way to specify '1'.

Change the nr_pages parameter to the explicit value of '1'.
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210525172428.3634316-3-ira.weiny@intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

fs/dax.c

@@ -721,7 +721,7 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
 		return rc;
 
 	id = dax_read_lock();
-	rc = dax_direct_access(dax_dev, pgoff, PHYS_PFN(PAGE_SIZE), &kaddr, NULL);
+	rc = dax_direct_access(dax_dev, pgoff, 1, &kaddr, NULL);
 	if (rc < 0) {
 		dax_read_unlock(id);
 		return rc;