gro: Check for GSO packets and packets with frag_list

As GRO cannot be applied to packets with frag_list we need to
make sure that we reject such packets if they are fed to us,
e.g., through a tunnel device.

Also there is no point in applying GRO on GSO packets so they
too should be rejected.  This allows GRO to be used in virtio-net
which may produce GSO packets directly but may still benefit
from GRO if the other end of it doesn't support GSO.

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