virtio-network: no bail-out on CWR even without host support

[rscheff]

Summary

Support for RFC3168 ECN with TSO is variably broken.

Worst scenario is to bail-out on encountering a CWR flag when
processing TSO on "unsupporting" host hardware. Expected
semantic changes alleviate the need to deal with CWR in any
special way in the TSO path (AccECN), while claimed RFC3168 ECN
support is frequently incorrect too.

See D43166 where dealing with RFC3168 CWR is moved entirely
to the tcp_output() function, bypassing TSO. Preparing
virtio to handle TSO with AccECN properly.

MFC after: 2 weeks

Test Plan

The TSO codepath should copy over all 12 TCP header flags,
with the exception of PSH and FIN, to all individual packets.