When a skb has multiple vlans and it is CHECKSUM_PARTIAL,
ixgbevf_tx_csum() fails to get the network protocol and checksum related
descriptor fields are not configured correctly because skb->protocol
doesn't show the L3 protocol in this case.

Use first->protocol instead of skb->protocol to get the proper network
protocol.
Signed-off-by: NToshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>