net/sfc: rework flow action RSS support

net/sfc: rework flow action RSS support

Currently, the driver always allocates a dedicated NIC RSS context
for every separate flow rule with action RSS, which is not optimal.

First of all, multiple rules which have the same RSS specification
can share a context since filters in the hardware operate this way.

Secondly, entries in a context's indirection table are not precise
queue IDs but offsets with regard to the base queue ID of a filter.
This way, for example, queue arrays "0, 1, 2" and "3, 4, 5" in two
otherwise identical RSS specifications allow the driver to use the
same context since they both yield the same table of queue offsets.

Rework flow action RSS support in order to use these optimisations.

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