upon router-interface-delete IPv4 subnet, all IPv6 entries disappear too

this is a dual stack scenario that has been found in relationship with 9138 issue.

scenario:

neutron net-create N1
neutron subnet-create --name V6N1 --ip-version 6 --ipv6-ra-mode slaac --ipv6-address-mode slaac N1 2001:db8:0:2::/64
neutron router-create router1
neutron router-interface-add router1 V6N1
neutron subnet-create --name V4N1 N1 15.0.0.0/24
neutron router-interface-add router1 V4N1
neutron port-create ... N1
=> fib-show should display both entries

neutron router-interface-delete router1 V4N1

=> fib-show should display only IPv6 entries.