pty: Fix locking bug on error path

We end up dropping the mutex twice on some errors. We don't want to do
that.

Reported-by: Fengguang Wu <[e-mail]>
Signed-off-by: Alan Cox <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/pty.c

···
 	index = devpts_new_index(inode);
 	if (index < 0) {
 		retval = index;
+		mutex_unlock(&devpts_mutex);
 		goto err_file;
 	}
 
···
 	mutex_unlock(&tty_mutex);
 	devpts_kill_index(inode, index);
 err_file:
-        mutex_unlock(&devpts_mutex);
 	tty_free_file(filp);
 	return retval;
 }