USB: add rationale on why usb descriptor structures have to be packed

/*

 * This file holds USB constants and structures that are needed for USB

 * device APIs.  These are used by the USB device model, which is defined

 * in chapter 9 of the USB 2.0 specification.  Linux has several APIs in C

 * that need these:

 * This file holds USB constants and structures that are needed for

 * USB device APIs.  These are used by the USB device model, which is

 * defined in chapter 9 of the USB 2.0 specification and in the

 * Wireless USB 1.0 (spread around).  Linux has several APIs in C that

 * need these:

 *

 * - the master/host side Linux-USB kernel driver API;

 * - the "usbfs" user space API; and

 *

 * There's also "Wireless USB", using low power short range radios for

 * peripheral interconnection but otherwise building on the USB framework.

 *

 * Note all descriptors are declared '__attribute__((packed))' so that:

 *

 * [a] they never get padded, either internally (USB spec writers

 *     probably handled that) or externally;

 *

 * [b] so that accessing bigger-than-a-bytes fields will never

 *     generate bus errors on any platform, even when the location of

 *     its descriptor inside a bundle isn't "naturally aligned", and

 *

 * [c] for consistency, removing all doubt even when it appears to

 *     someone that the two other points are non-issues for that

 *     particular descriptor type.

 */

#ifndef __LINUX_USB_CH9_H