net/sched: act_ct: Offload only ASSURED connections

Short-lived connections increase the insertion rate requirements,
fill the offload table and provide very limited offload value since
they process a very small amount of packets. The ct ASSURED flag is
designed to filter short-lived connections for early expiration.

Offload connections when they are ESTABLISHED and ASSURED.

Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>