dax: Ensure errno is returned from dax_direct_access

If the caller specifies a negative nr_pages that is an invalid
parameter.

Return -EINVAL to ensure callers get an errno if they want to check it.
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210525172428.3634316-4-ira.weiny@intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/super.c

@@ -313,7 +313,7 @@ long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		return -ENXIO;
 
 	if (nr_pages < 0)
-		return nr_pages;
+		return -EINVAL;
 
 	avail = dax_dev->ops->direct_access(dax_dev, pgoff, nr_pages,
 			kaddr, pfn);