ethtool: add ethtool_rx_flow_spec to flow_rule structure translator

This patch adds a function to translate the ethtool_rx_flow_spec
structure to the flow_rule representation.

This allows us to reuse code from the driver side given that both flower
and ethtool_rx_flow interfaces use the same representation.

This patch also includes support for the flow type flags FLOW_EXT,
FLOW_MAC_EXT and FLOW_RSS.

The ethtool_rx_flow_spec_input wrapper structure is used to convey the
rss_context field, that is away from the ethtool_rx_flow_spec structure,
and the ethtool_rx_flow_spec structure.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>