Redefine two TCP header functions to accept TCP header pointer.
When subtracting, return signed int to allow error checking.

In the only two existing files using the latter function, clean up
confusing and inconsistent mixing of both byte and word offsets.
However, remove proposed header length checking, and document the
assumptions instead.  In the immortal words of the reviewer:

   This is transmit, and the packets can only come from the Linux
   TCP stack, not some external entity.

   You're being way too anal here, and adding these checks to
   drivers would be just a lot of rediculious bloat. [sic]

These functions will also be used in subsequent patches that implement
additional features.
---
  drivers/net/bnx2.c  |   22 +++++++++++++---------
  drivers/net/tg3.c   |   32 +++++++++++++++++---------------
  include/linux/tcp.h |   10 ++++++++--
  3 files changed, 38 insertions(+), 26 deletions(-)