ixgbevf: Fix checksum error when using stacked vlan

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>

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c

@@ -3099,7 +3099,7 @@ static int ixgbevf_tso(struct ixgbevf_ring *tx_ring,
 	/* ADV DTYP TUCMD MKRLOC/ISCSIHEDLEN */
 	type_tucmd = IXGBE_ADVTXD_TUCMD_L4T_TCP;
 
-	if (skb->protocol == htons(ETH_P_IP)) {
+	if (first->protocol == htons(ETH_P_IP)) {
 		struct iphdr *iph = ip_hdr(skb);
 		iph->tot_len = 0;
 		iph->check = 0;
@@ -3156,7 +3156,7 @@ static void ixgbevf_tx_csum(struct ixgbevf_ring *tx_ring,
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		u8 l4_hdr = 0;
-		switch (skb->protocol) {
+		switch (first->protocol) {
 		case htons(ETH_P_IP):
 			vlan_macip_lens |= skb_network_header_len(skb);
 			type_tucmd |= IXGBE_ADVTXD_TUCMD_IPV4;