net/sfc: make flow list engine-agnostic

net/sfc: make flow list engine-agnostic

A backend which a driver employs to handle flow rules of a given
type depends on the underlying NIC flow engine. The driver in
question in its current state is tailored to support the only
flow engine, VNIC filtering. As the need arises to add support
for transfer rules, the driver has to be reworked so that it
becomes possible to introduce yet another backend.

As a preparation step, make the flow list shared
between different engines.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>