geneve: Use GRO cells infrastructure.

Geneve can benefit from GRO at the device level in a manner similar
to other tunnels, especially as hardware offloads are still emerging.

After this patch, aggregated frames are seen on the tunnel interface.
Single stream throughput nearly doubles in ideal circumstances (on
old hardware).

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>