net/sfc: add infrastructure to make many filters from flow

net/sfc: add infrastructure to make many filters from flow

Not all flow rules can be expressed in one hardware filter, so some flow
rules have to be expressed in terms of multiple hardware filters. This
patch provides a means to produce a filter spec template from the flow
rule which then can be used to produce a set of fully elaborated specs
to be inserted.

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