net/sfc: add verify method to flow validate path

net/sfc: add verify method to flow validate path

The new method is needed to make sure that a flow being
validated will have a chance to be accepted by the FW.
MAE-specific implementation of the method should
compare the class of a rule being validated with
the corresponding classes of active rules, and,
if no matches found, make a request to the FW.
Support for the latter will be added in future.

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