If the virtio network driver is not told by the host
that proper ECN support is available with TSO, it returns
ENOTSUP. This is not dealt with other than an effective
packet discard - and subsequently, a CWR-flagged TSO
transmission attempt is made until RTO reduces the
congestion window to a single packet.
Split the CWR-flagged TSO transmission in this case,
and resend immediately with the first segment with CWR
as non-TSO.