igbvf: fix truesize underestimation

igbvf 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>
CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igbvf/netdev.c

@@ -312,7 +312,7 @@ static bool igbvf_clean_rx_irq(struct igbvf_adapter *adapter,
 
 			skb->len += length;
 			skb->data_len += length;
-			skb->truesize += length;
+			skb->truesize += PAGE_SIZE / 2;
 		}
 send_up:
 		i++;