tty: USB hangup is racy

commit d2b39182 upstream.

The USB layer uses tty_hangup to deal with unplugs of the physical hardware
(analogous to loss of carrier) and then frees the resources. However the
tty_hangup is asynchronous. As the hangup can sleep we can use tty_vhangup
which is the non async version to avoid freeing resources too early.
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -1161,10 +1161,7 @@ void usb_serial_disconnect(struct usb_interface *interface)
 		if (port) {
 			struct tty_struct *tty = tty_port_tty_get(&port->port);
 			if (tty) {
-				/* The hangup will occur asynchronously but
-				   the object refcounts will sort out all the
-				   cleanup */
-				tty_hangup(tty);
+				tty_vhangup(tty);
 				tty_kref_put(tty);
 			}
 			kill_traffic(port);