net: fib_rules: Enable flow label selector usage

Now that both IPv4 and IPv6 correctly handle the new flow label
attributes, enable user space to configure FIB rules that make use of
the flow label by changing the policy to stop rejecting them and
accepting 32 bit values in big-endian byte order.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>