gk/fib.c:check_gateway_prefix() is too strict

[AltraMayor]

The default/example configuration sets 10.0.1.1/24 as the front IPv4 network. Therefore, 10.0.1.2 is a neighbor. However, adding the routing 10.0.0.0/8 with gateway 10.0.1.2 fails because gk/fib.c:check_gateway_prefix() concludes that DYC/1: Gateway address is in prefix 10.0.0.0/8, so gateway is not a neighbor.