net: thunderbolt: Fix TCPv6 GSO checksum calculation

Alex reported that running ssh over IPv6 does not work with
Thunderbolt/USB4 networking driver. The reason for that is that driver
should call skb_is_gso() before calling skb_is_gso_v6(), and it should
not return false after calculates the checksum successfully. This probably
was a copy paste error from the original driver where it was done properly.
Reported-by: Alex Balcanquall <alex@alexbal.com>
Fixes: e69b6c02 ("net: Add support for networking over Thunderbolt cable")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/thunderbolt/main.c

@@ -1049,12 +1049,11 @@ static bool tbnet_xmit_csum_and_map(struct tbnet *net, struct sk_buff *skb,
 		*tucso = ~csum_tcpudp_magic(ip_hdr(skb)->saddr,
 					    ip_hdr(skb)->daddr, 0,
 					    ip_hdr(skb)->protocol, 0);
-	} else if (skb_is_gso_v6(skb)) {
+	} else if (skb_is_gso(skb) && skb_is_gso_v6(skb)) {
 		tucso = dest + ((void *)&(tcp_hdr(skb)->check) - data);
 		*tucso = ~csum_ipv6_magic(&ipv6_hdr(skb)->saddr,
 					  &ipv6_hdr(skb)->daddr, 0,
 					  IPPROTO_TCP, 0);
-		return false;
 	} else if (protocol == htons(ETH_P_IPV6)) {
 		tucso = dest + skb_checksum_start_offset(skb) + skb->csum_offset;
 		*tucso = ~csum_ipv6_magic(&ipv6_hdr(skb)->saddr,