[SK_BUFF]: Convert skb->end to sk_buff_data_t

Now to convert the last one, skb->data, that will allow many simplifications
and removal of some of the offset helpers.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/usb/atm/usbatm.c b/drivers/usb/atm/usbatm.c
index 4d8f282..a076f73 100644
--- a/drivers/usb/atm/usbatm.c
+++ b/drivers/usb/atm/usbatm.c
@@ -335,12 +335,12 @@
 
 	sarb = instance->cached_vcc->sarb;
 
-	if (skb_tail_pointer(sarb) + ATM_CELL_PAYLOAD > sarb->end) {
+	if (sarb->tail + ATM_CELL_PAYLOAD > sarb->end) {
 		atm_rldbg(instance, "%s: buffer overrun (sarb->len %u, vcc: 0x%p)!\n",
 				__func__, sarb->len, vcc);
 		/* discard cells already received */
 		skb_trim(sarb, 0);
-		UDSL_ASSERT(skb_tail_pointer(sarb) + ATM_CELL_PAYLOAD <= sarb->end);
+		UDSL_ASSERT(sarb->tail + ATM_CELL_PAYLOAD <= sarb->end);
 	}
 
 	memcpy(skb_tail_pointer(sarb), source + ATM_CELL_HEADER, ATM_CELL_PAYLOAD);