USB: export <linux/usb_gadgetfs> as <linux/usb/gadgetfs.h>

Make sure gadgetfs userspace interface is properly exported:

 - Move <linux/usb_gadgetfs.h> to <linux/usb/gadgetfs.h>;
 - Export it using Kbuild;
 - Add an #include guard;
 - Correct some internal documentation;
 - Update struct layout so it's the same on 32/64 bit kernels.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>