net: ethtool: Unify ETHTOOL_{G,S}RXFH rxnfc copy

ETHTOOL_GRXFH correctly copies in the full struct ethtool_rxnfc when
FLOW_RSS is set; ETHTOOL_SRXFH needs a similar code path to handle the
FLOW_RSS case so that ethtool can set the flow hash for custom RSS
contexts (if supported by the driver).

The copy code from ETHTOOL_GRXFH has been pulled out in to a helper so
that it can be called in both ETHTOOL_{G,S}RXFH code paths.

Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: David S. Miller <davem@davemloft.net>