[NET]: Kill the WARN_ON() calls for checksum fixups.

We have a more complete solution in the works, involving
the seperation of CHECKSUM_HW on input vs. output, and
having netfilter properly do incremental checksums.

But that is a very involved patch and is thus 2.6.19
material.

What we have now is infinitely better than the past,
wherein all TSO packets were dropped due to corrupt
checksums as soon at the NAT module was loaded.  At
least now, the checksums do get fixed up, it just
isn't the cleanest nor most optimal solution.

Signed-off-by: David S. Miller <davem@davemloft.net>