net/sched: act_ct: Fill offloading tuple iifidx

Driver offloading ct tuples can use the information of which devices
received the packets that created the offloaded connections, to
more efficiently offload them only to the relevant device.

Add new act_ct nf conntrack extension, which is used to store the skb
devices before offloading the connection, and then fill in the tuple
iifindex so drivers can get the device via metadata dissector match.

Signed-off-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>