cdc-acm: fix race between reset and control messaging

If a device splits up a control message and a reset() happens
between the parts, the message is lost and already recieved parts
must be dropped.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Fixes: 1aba579f3c ("cdc-acm: handle read pipe errors")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -1642,6 +1642,7 @@ static int acm_pre_reset(struct usb_interface *intf)
 	struct acm *acm = usb_get_intfdata(intf);
 
 	clear_bit(EVENT_RX_STALL, &acm->flags);
+	acm->nb_index = 0; /* pending control transfers are lost */
 
 	return 0;
 }