vtnet: fix TSO for TCP/IPv6

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 fix is instantly MFCed.

Approved by: re(gjb)
PR: 254366
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D29331

(cherry picked from commit d4697a6b56168876fc0ffec1a0bb1b24d25b198e)