net/sfc: add actions parsing stub to MAE backend

net/sfc: add actions parsing stub to MAE backend

If parsing a flow results in an action set specification
identical to an already existing one, duplication will
be avoided by reusing the list entry of the latter.
Using an attach helper and a reference counter
is meant to serve the said purpose.

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