net: Use skb_frag_off accessors

Use accessor functions for skb fragment's page_offset instead
of direct references, in preparation for bvec conversion.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/niu.c

@@ -6695,7 +6695,7 @@ static netdev_tx_t niu_start_xmit(struct sk_buff *skb,
 
 		len = skb_frag_size(frag);
 		mapping = np->ops->map_page(np->device, skb_frag_page(frag),
-					    frag->page_offset, len,
+					    skb_frag_off(frag), len,
 					    DMA_TO_DEVICE);
 
 		rp->tx_buffs[prod].skb = NULL;