ftmac100: fix skb truesize underestimation

ftmac100 allocates a page per skb fragment. We must account
PAGE_SIZE increments on skb->truesize, not the actual frag length.

If frame is under 64 bytes, page is freed, so increase truesize only for
bigger frames.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Po-Yu Chuang <ratbert@faraday-tech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/faraday/ftmac100.c

@@ -439,7 +439,10 @@ static bool ftmac100_rx_packet(struct ftmac100 *priv, int *processed)
 	skb_fill_page_desc(skb, 0, page, 0, length);
 	skb->len += length;
 	skb->data_len += length;
-	skb->truesize += length;
+
+	/* page might be freed in __pskb_pull_tail() */
+	if (length > 64)
+		skb->truesize += PAGE_SIZE;
 	__pskb_pull_tail(skb, min(length, 64));
 
 	ftmac100_alloc_rx_page(priv, rxdes, GFP_ATOMIC);