net/mlx5e: Fix features check of IPv6 traffic

[ Upstream commit 2989ad1e ]

The assumption that the next header field contains the transport
protocol is wrong for IPv6 packets with extension headers.
Instead, we should look the inner-most next header field in the buffer.
This will fix TSO offload for tunnels over IPv6 with extension headers.

Performance testing: 19.25x improvement, cool!
Measuring bandwidth of 16 threads TCP traffic over IPv6 GRE tap.
CPU: Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz
NIC: Mellanox Technologies MT28800 Family [ConnectX-5 Ex]
TSO: Enabled
Before: 4,926.24  Mbps
Now   : 94,827.91 Mbps

Fixes: b3f63c3d ("net/mlx5e: Add netdev support for VXLAN tunneling")
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/ethernet/mellanox/mlx5/core/en_main.c

@@ -3038,6 +3038,7 @@ static netdev_features_t mlx5e_vxlan_features_check(struct mlx5e_priv *priv,
 						    struct sk_buff *skb,
 						    netdev_features_t features)
 {
+	unsigned int offset = 0;
 	struct udphdr *udph;
 	u16 proto;
 	u16 port = 0;
@@ -3047,7 +3048,7 @@ static netdev_features_t mlx5e_vxlan_features_check(struct mlx5e_priv *priv,
 		proto = ip_hdr(skb)->protocol;
 		break;
 	case htons(ETH_P_IPV6):
-		proto = ipv6_hdr(skb)->nexthdr;
+		proto = ipv6_find_hdr(skb, &offset, -1, NULL, NULL);
 		break;
 	default:
 		goto out;