MFC r265942:
  Fix checksum computation.  Previously it didn't include carry.
MFC r265943:
  Disable TX IP/TCP/UDP checksum offloading for RTL8168C/RTL8168CP.
  Previously only TX IP checksum offloading was disabled but it's
  reported that TX checksum offloading for UDP datagrams with IP
  options also generates corrupted frames.  Reporter's controller is
  RTL8168CP but I guess RTL8168C also have the same issue since it
  shall share the same core.