The decision whether a TCP packet is sent over IPv4 or IPv6 was based on ethertype, which works correctly.
In D27926 the criteria was changed to checking if the CSUM_IP_TSO flag is set in the csum-flags and then considering it to be TCP/IPv4.
However, the TCP stack sets the flag to CSUM_TSO for IPv4 and IPv6, where CSUM_TSO is defined as CSUM_IP_TSO|CSUM_IP6_TSO.
Therefore TCP/IPv6 packets gets mis-classified as TCP/IPv4, which breaks TSO for TCP/IPv6.
This patch bases the check again on the ethertype.
This fixes the bug reported in PR254366.