[NET]: Warn about GSO/checksum abuse

Now that Patrick has added the code to deal with GSO in netfilter,
we no longer need the crutch that computes partial checksums just
before transmission.

This patch turns this into a warning again.  If this goes OK, we
can then turn it into a BUG_ON and remove the gso_send_check cruft.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>