net/hyperv: rx_bytes should account the ether header size

skb->len after call eth_type_trans() does not include the ether
header size, but rx_bytes should account it.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>