[Bluetooth] Hangup TTY before releasing rfcomm_dev

The core problem is that RFCOMM socket layer ioctl can release
rfcomm_dev struct while RFCOMM TTY layer is still actively using
it. Calling tty_vhangup() is needed for a synchronous hangup before
rfcomm_dev is freed.

Addresses the oops at http://bugzilla.kernel.org/show_bug.cgi?id=7509

Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/rfcomm/tty.c

@@ -383,6 +383,10 @@ static int rfcomm_release_dev(void __user *arg)
 	if (req.flags & (1 << RFCOMM_HANGUP_NOW))
 		rfcomm_dlc_close(dev->dlc, 0);
 
+	/* Shut down TTY synchronously before freeing rfcomm_dev */
+	if (dev->tty)
+		tty_vhangup(dev->tty);
+
 	rfcomm_dev_del(dev);
 	rfcomm_dev_put(dev);
 	return 0;