nfp: flower: compute link aggregation action

If the egress device of an offloaded rule is a LAG port, then encode the
output port to the NFP with a LAG identifier and the offloaded group ID.

A prelag action is also offloaded which must be the first action of the
series (although may appear after other pre-actions - e.g. tunnels). This
causes the FW to check that it has the necessary information to output to
the requested LAG port. If it does not, the packet is sent to the kernel
before any other actions are applied to it.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>