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

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -7227,11 +7227,11 @@ netdev_tx_t ixgbe_xmit_frame_ring(struct sk_buff *skb,
 		if (!vhdr)
 			goto out_drop;
 
-		protocol = vhdr->h_vlan_encapsulated_proto;
 		tx_flags |= ntohs(vhdr->h_vlan_TCI) <<
 				  IXGBE_TX_FLAGS_VLAN_SHIFT;
 		tx_flags |= IXGBE_TX_FLAGS_SW_VLAN;
 	}
+	protocol = vlan_get_protocol(skb);
 
 	if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
 	    adapter->ptp_clock &&