sfxge(4): add information if TSO workaround is required

sfxge(4): add information if TSO workaround is required

In SF bug 61297 it's been confirmed that the hardware does not always
calculate the TCP checksum correctly with TSO sends.

The value of the Total Length field (IPv4) or Payload Length field
(IPv6) is the critical factor. We're sufficiently confident that if
these fields are zero then the checksum will be calculated correctly.

The information may be used by the drivers to check if the workaround is
required when FATSOv2 is implemented.

Submitted by:   Mark Spender <mspender at solarflare.com>
Sponsored by:   Solarflare Communications, Inc.
Differential Revision:  https://reviews.freebsd.org/D18258