cxgb3: Replace LRO with GRO

This patch makes cxgb3 invoke the GRO hooks instead of LRO.  As
GRO has a compatible external interface to LRO this is a very
straightforward replacement.

I've kept the ioctl controls for per-queue LRO switches.  However,
we should not encourage anyone to use these.

Because of that, I've also kept the skb construction code in
cxgb3.  Hopefully we can phase out those per-queue switches
and then kill this too.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>