usbatm_control
if (sizeof(struct usbatm_control) > sizeof_field(struct sk_buff, cb)) {
#define UDSL_SKB(x) ((struct usbatm_control *)(x)->cb)
struct usbatm_control *ctrl = UDSL_SKB(skb);
struct usbatm_control *ctrl = UDSL_SKB(skb);