- reset ift_npending after flush
Further accomodations for cxgb:
- Make timer shared context calls be per txq
- Make timeouts per-cpu
- handle case where encap call frees the mbuf due to I/O completion
  (cxgb does PIO on tx for packets len <= 96 bytes
- handle case of multiple packets per tx descriptor
- handle case where encap keeps the packet but doesn't actually
  send it immediately (at the latest will be sent in txd_flush)

Among other things, still need to actually find a more sensible
setting for per-cpu timers than idx % mp_ncpus