atm: idt77252: fix build broken on amd64

idt77252 is broken and wont load on amd64 systems
modprobe idt77252 shows the following

idt77252_init: skb->cb is too small (48 < 56)

Add packed attribute to struct idt77252_skb_prv and struct atm_skb_data
so that the total size can be <= sizeof(skb->cb)
Also convert runtime size check to buildtime size check in
idt77252_init()

Signed-off-by: Tong Zhang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/atm/idt77252.c

···
 	struct sk_buff *skb;
 
 	printk("%s: at %p\n", __func__, idt77252_init);
-
-	if (sizeof(skb->cb) < sizeof(struct atm_skb_data) +
-			      sizeof(struct idt77252_skb_prv)) {
-		printk(KERN_ERR "%s: skb->cb is too small (%lu < %lu)\n",
-		       __func__, (unsigned long) sizeof(skb->cb),
-		       (unsigned long) sizeof(struct atm_skb_data) +
-				       sizeof(struct idt77252_skb_prv));
-		return -EIO;
-	}
-
+	BUILD_BUG_ON(sizeof(skb->cb) < sizeof(struct idt77252_skb_prv) + sizeof(struct atm_skb_data));
 	return pci_register_driver(&idt77252_driver);
 }
 

drivers/atm/idt77252.h

···
 	struct scqe	tbd;	/* Transmit Buffer Descriptor */
 	dma_addr_t	paddr;	/* DMA handle */
 	u32		pool;	/* sb_pool handle */
-};
+} __packed;
 
 #define IDT77252_PRV_TBD(skb)	\
 	(((struct idt77252_skb_prv *)(ATM_SKB(skb)+1))->tbd)

include/linux/atmdev.h

···
 	struct atm_vcc	*vcc;		/* ATM VCC */
 	unsigned long	atm_options;	/* ATM layer options */
 	unsigned int	acct_truesize;  /* truesize accounted to vcc */
-};
+} __packed;
 
 #define VCC_HTABLE_SIZE 32