ice: Fix tunnel checksum offload with fragmented traffic

Fix checksum offload on VXLAN tunnels.
In case, when mpls protocol is not used, set l4 header to transport
header of skb. This fixes case, when user tries to offload checksums
of VXLAN tunneled traffic.

Steps for reproduction (requires link partner with tunnels):
ip l s enp130s0f0 up
ip a f enp130s0f0
ip a a 10.10.110.2/24 dev enp130s0f0
ip l s enp130s0f0 mtu 1600
ip link add vxlan12_sut type vxlan id 12 group 238.168.100.100 dev enp130s0f0 dstport 4789
ip l s vxlan12_sut up
ip a a 20.10.110.2/24 dev vxlan12_sut
iperf3 -c 20.10.110.1 #should connect

Offload params: td_offset, cd_tunnel_params were
corrupted, due to l4 header pointing wrong address. NIC would then drop
those packets internally, due to incorrect TX descriptor data,
which increased GLV_TEPC register.

Fixes: 69e66c04 ("ice: Add mpls+tso support")
Signed-off-by: Przemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_txrx.c

@@ -1751,11 +1751,13 @@ int ice_tx_csum(struct ice_tx_buf *first, struct ice_tx_offload_params *off)
 
 	protocol = vlan_get_protocol(skb);
 
-	if (eth_p_mpls(protocol))
+	if (eth_p_mpls(protocol)) {
 		ip.hdr = skb_inner_network_header(skb);
-	else
+		l4.hdr = skb_checksum_start(skb);
+	} else {
 		ip.hdr = skb_network_header(skb);
-	l4.hdr = skb_checksum_start(skb);
+		l4.hdr = skb_transport_header(skb);
+	}
 
 	/* compute outer L2 header size */
 	l2_len = ip.hdr - skb->data;