igb: Replace LRO with GRO

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

Three things of note:

1) I've kept the LRO Kconfig option until we decide to enable
GRO across the board at which point it can also be killed.

2) The poll_controller stuff is broken in igb as it tries to do
the same work as the normal poll routine.  Since poll_controller
can be called in the middle of a poll, this can't be good.

I noticed this because poll_controller can invoke the GRO hooks
without flushing held GRO packets.

However, this should be harmless (assuming the poll_controller
bug above doesn't kill you first :) since the next ->poll will
clear the backlog.  The only time when we'll have a problem is
if we're already executing the GRO code on the same ring, but
that's no worse than what happens now.

3) I kept the ip_summed check before calling GRO so that we're
on par with previous behaviour.

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