selftests: drv-net: add test for RSS on flow label

selftests: drv-net: add test for RSS on flow label

Add a simple test for checking that RSS on flow label works,
and that its rejected for IPv4 flows.

 # ./tools/testing/selftests/drivers/net/hw/rss_flow_label.py
 TAP version 13
 1..2
 ok 1 rss_flow_label.test_rss_flow_label
 ok 2 rss_flow_label.test_rss_flow_label_6only
 # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20250811234212.580748-5-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>