net: flow_offload: provision conntrack info in ct_metadata

In order to offload connections in other states besides "established" the
driver offload callbacks need to have access to connection conntrack info.
Flow offload intermediate representation data structure already contains
that data encoded in 'cookie' field, so just reuse it in the drivers.

Reject offloading IP_CT_NEW connections for now by returning an error in
relevant driver callbacks based on value of ctinfo. Support for offloading
such connections will need to be added to the drivers afterwards.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>