ifb: support more features

When using ifb+netem on ingress on SIT/IPIP/GRE traffic,
GRO packets are not properly processed.

Segmentation should not be forced, since ifb is already adding
quite a performance hit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>