net/sched: act_ct: offload UDP NEW connections

Modify the offload algorithm of UDP connections to the following:

- Offload NEW connection as unidirectional.

- When connection state changes to ESTABLISHED also update the hardware
flow. However, in order to prevent act_ct from spamming offload add wq for
every packet coming in reply direction in this state verify whether
connection has already been updated to ESTABLISHED in the drivers. If that
it the case, then skip flow_table and let conntrack handle such packets
which will also allow conntrack to potentially promote the connection to
ASSURED.

- When connection state changes to ASSURED set the flow_table flow
NF_FLOW_HW_BIDIRECTIONAL flag which will cause refresh mechanism to offload
the reply direction.

All other protocols have their offload algorithm preserved and are always
offloaded as bidirectional.

Note that this change tries to minimize the load on flow_table add
workqueue. First, it tracks the last ctinfo that was offloaded by using new
flow 'NF_FLOW_HW_ESTABLISHED' flag and doesn't schedule the refresh for
reply direction packets when the offloads have already been updated with
current ctinfo. Second, when 'add' task executes on workqueue it always
update the offload with current flow state (by checking 'bidirectional'
flow flag and obtaining actual ctinfo/cookie through meta action instead of
caching any of these from the moment of scheduling the 'add' work)
preventing the need from scheduling more updates if state changed
concurrently while the 'add' work was pending on workqueue.

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