MFC r235961:

MFC r235961:

   Add code to handle pre-checked TCP checksums as indicated by mbuf
   flags to save the entire computation for validation if not needed.

   In the IPv6 TCP output path only compute the pseudo-header checksum,
   set the checksum offset in the mbuf field along the appropriate flag
   as done in IPv4.

   In tcp_respond() just initialize the IPv6 payload length to 0 as
   ip6_output() will properly set it.

Approved by:	re

git-svn-id: svn://svn.freebsd.org/base/stable/9@238237 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f