MFC r263957:
  Increase the number of TX DMA segments from 32 to 35.  It turned
  out 32 is not enough to support a full sized TSO packet.
  While I'm here fix a long standing bug introduced in r169632 in
  bce(4) where it didn't include L2 header length of TSO packet in
  the maximum DMA segment size calculation.
MFC r259543:
  Failed m_devget(9) indicates lack of free mbuf cluster.  Update
  if_iqdrops counter for that case since the received frame is ok.
  While here, simplify updating counter logic.
MFC r260429:
  m_defrag(9) does not touch original mbuf chain when it can't
  allocate new mbuf.  Free original mbuf chain when driver is not
  able to send the packet.