can: rework skb reserved data handling

Added accessor and skb_reserve helpers for struct can_skb_priv.
Removed pointless skb_headroom() check.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
CC: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/can/gw.c

@@ -381,9 +381,7 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 
 	/* is sending the skb back to the incoming interface not allowed? */
 	if (!(gwj->flags & CGW_FLAGS_CAN_IIF_TX_OK) &&
-	    skb_headroom(skb) == sizeof(struct can_skb_priv) &&
-	    (((struct can_skb_priv *)(skb->head))->ifindex ==
-	     gwj->dst.dev->ifindex))
+	    can_skb_prv(skb)->ifindex == gwj->dst.dev->ifindex)
 		return;
 
 	/*