sfc: Support ARFS for IPv6 flows

Extend efx_filter_rfs() to map TCP/IPv6 and UDP/IPv6 flows into
efx_filter_spec.  These are only supported on EF10; on Falcon and
Siena they will be rejected by efx_farch_filter_from_gen_spec().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>