selftests: drv-net: rss_ctx: test dumping RSS contexts

Add a test for dumping RSS contexts. Make sure indir table
and key are sane when contexts are created with various
combination of inputs. Test the dump filtering by ifname
and start-context.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>