igb: fix skb truesize underestimation

e1000 allocates half a page per skb fragment. We must account
PAGE_SIZE/2 increments on skb->truesize, not the actual frag length.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -5950,7 +5950,7 @@ static bool igb_clean_rx_irq(struct igb_q_vector *q_vector, int budget)
 
 			skb->len += length;
 			skb->data_len += length;
-			skb->truesize += length;
+			skb->truesize += PAGE_SIZE / 2;
 
 			if ((page_count(buffer_info->page) != 1) ||
 			    (page_to_nid(buffer_info->page) != current_node))