eth: fbnic: support RSS on IPv6 Flow Label

eth: fbnic: support RSS on IPv6 Flow Label

Support IPv6 Flow Label hashing. Use both inner and outer IPv6
header's Flow Label if both headers are detected. Flow Label
is unlike normal header fields, by enabling it user accepts
the unstable hash and possible reordering. Because of that
I think it's reasonable to hash over all Flow Labels we can
find, even tho we don't hash over all L3 addresses.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250811234212.580748-3-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>