MFp4 bz_ipv6_fast:

MFp4 bz_ipv6_fast:

  Optimize in6_cksum(), re-ordering work and limiting variable
  initialization, removing a bzero() for mostly re-initialized
  struct values, making use of the newly introduced in6_getscope(),
  as well as converting an if/panic to a KASSERT().

  Sponsored by:	The FreeBSD Foundation
  Sponsored by:	iXsystems

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