mlxsw: spectrum_router: Extract mlxsw_sp_fi_is_gateway()

mlxsw: spectrum_router: Extract mlxsw_sp_fi_is_gateway()

For IPv4 IP-in-IP offload, routes that direct traffic to IP-in-IP
devices need to be considered gateway routes as well. That involves a
bit more logic, so extract the current test to a separate function,
where the logic can be later added.

Signed-off-by: Petr Machata <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Signed-off-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>