Harmonize tcp_v4_rcv() and tcp_v6_rcv() -- better document tcp doff and
header length assumptions.

Reduces multiply/shifts, marginally improving speed.

Retains redundant tcp header length checks before checksumming.

Stand-alone patch, originally developed for TCPCT.

Requires:
   net: tcp_header_len_th and tcp_option_len_th

Signed-off-by: William.Allen.Simpson@gmail.com
---
  net/ipv4/tcp_ipv4.c |   36 +++++++++++++++++++-------------
  net/ipv6/tcp_ipv6.c |   56 ++++++++++++++++++++++++++++----------------------
  2 files changed, 52 insertions(+), 40 deletions(-)