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 <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>