MFp4 bz_ipv6_fast:

MFp4 bz_ipv6_fast:

  Introduce a (for now copied stripped down) in6_cksum_pseudo()
  function.  We should be able to use this from in6_cksum() but
  we should also ponder possible MD specific improvements.
  It takes an extra csum argument to allow for easy checks as
  will be done by the upper layer protocol input paths.

  Sponsored by:	The FreeBSD Foundation
  Sponsored by:	iXsystems

Reviewed by:	gnn (as part of the whole)
MFC After:	3 days