net/mlx5e: Fix loopback self test when GRO is off

When GRO is off, the transport header pointer in sk_buff is
initialized to network's header.

To find the udp header, instead of using udp_hdr() which assumes
skb_network_header was set, manually calculate the udp header offset.

Fixes: 0952da79 ("net/mlx5e: Add support for loopback selftest")
Signed-off-by: NInbar Karmy <inbark@mellanox.com>
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>

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

@@ -216,7 +216,8 @@ mlx5e_test_loopback_validate(struct sk_buff *skb,
 	if (iph->protocol != IPPROTO_UDP)
 		goto out;
 
-	udph = udp_hdr(skb);
+	/* Don't assume skb_transport_header() was set */
+	udph = (struct udphdr *)((u8 *)iph + 4 * iph->ihl);
 	if (udph->dest != htons(9))
 		goto out;