MFC r217246,217832:

MFC r217246,217832:
r217246:
  Implement TSO on RealTek RTL8168/8111 C or later controllers.
  RealTek changed TX descriptor format for later controllers so these
  controllers require MSS configuration in different location of TX
  descriptor. TSO is enabled by default for controllers that use new
  descriptor format.
  For old controllers, TSO is still disabled by default due to broken
  frames under certain conditions but users can enable it.
  Special thanks to Hayes Wang at RealTek.

r217832:
  Disable TSO for all Realtek controllers. Experimentation showed
  RTL8111C generated corrupted frames where TCP option header was
  broken. All other sample controllers I have did not show such
  problem so it could be RTL8111C specific issue. Because there are
  too many variants it's hard to tell how many controllers have such
  issue. Just disable TSO by default but have user override it.

git-svn-id: svn://svn.freebsd.org/base/stable/8@218897 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f