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/broadcom/bnxt/bnxt.c

@@ -957,7 +957,7 @@ static struct sk_buff *bnxt_rx_page_skb(struct bnxt *bp,
 
 	frag = &skb_shinfo(skb)->frags[0];
 	skb_frag_size_sub(frag, payload);
-	frag->page_offset += payload;
+	skb_frag_off_add(frag, payload);
 	skb->data_len -= payload;
 	skb->tail += payload;