cxgbe: Allow parse_pkt to internally queue a packet.

[jhb]

Summary

If parse_pkt returns EINPROGRESS, return from cxgbe_transmit
without queueing the packet in a txq. Use this to move the call
to ethofld_transmit for packet pacing into parse_pkt.

[np]

sys/dev/cxgbe/t4_main.c

3063–3070

Should we get rid of the __predict_false if EINPROGRESS can commonly occur on the fast path for tx?

[jhb]

sys/dev/cxgbe/t4_main.c

3063–3070

Hmmm, it depends on what we want to optimize for. Plain NIC traffic will still take the rc == 0 case. Also, for the EINPROGRESS case, all the work has already been done in parse_pkt and we are just trying to return after possibly draining the mp_ring, so I think that's still less important to optimize for vs getting down to the fast path for plain TX.