ixgbe: Fix checksum error when using stacked vlan

When a skb has multiple vlans and it is CHECKSUM_PARTIAL,
ixgbe_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 vlan_get_protocol() to get the proper network protocol.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>