net: ethtool: copy the rxfh flow handling

RX Flow Hash configuration uses the same argument structure
as flow filters. This is probably why ethtool IOCTL handles
them together. The more checks we add the more convoluted
this code is getting (as some of the checks apply only
to flow filters and others only to the hashing).

Copy the code to separate the handling. This is an exact
copy, the next change will remove unnecessary handling.

Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20250611145949.2674086-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>