[readdir] constify ->actor

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 237af62..7c30e3a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1507,7 +1507,7 @@
  */
 typedef int (*filldir_t)(void *, const char *, int, loff_t, u64, unsigned);
 struct dir_context {
-	filldir_t actor;
+	const filldir_t actor;
 	loff_t pos;
 };