pty: make ptmx file ops read-only after init

The ptmx_fops structure is only changed during init, so mark it as such.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -800,7 +800,7 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	return retval;
 }
 
-static struct file_operations ptmx_fops;
+static struct file_operations ptmx_fops __ro_after_init;
 
 static void __init unix98_pty_init(void)
 {