net/sfc: support group flows in tunnel offload

net/sfc: support group flows in tunnel offload

GROUP is an in-house term for so-called "tunnel_match" flows.
On parsing, they are detected by virtue of PMD-internal item
MARK. It associates a given flow with its tunnel context.

Such a flow is represented by a MAE action rule which is
chained with the corresponding JUMP rule's outer rule
by virtue of matching on its recirculation ID.

GROUP flows do narrower match than JUMP flows do and
decapsulate matching packets (full offload).

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>