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

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

This changeset is from upstream Linux.  We need it so Xen users
using NAT are not unnecessarily alarmed by these WARN_ON errors
especially since they're not in a position to do anything about
it.  Original changelog:

        [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>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>