xen/xenfs: set_page_dirty is supposed to return true if it dirties

I don't think it matters at all in this case (there's only one caller
which checks the return value), but may as well be strictly correct.

[ Impact: cleanup ]
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/xenfs/super.c

@@ -26,9 +26,7 @@ MODULE_LICENSE("GPL");
 
 static int xenfs_set_page_dirty(struct page *page)
 {
-	if (!PageDirty(page))
-		SetPageDirty(page);
-	return 0;
+	return !TestSetPageDirty(page);
 }
 
 static const struct address_space_operations xenfs_aops = {