lib: fix order of northbound callbacks

lib: fix order of northbound callbacks

When ordering the NB callbacks according to their priorities, if the
operation is "destroy" we should reverse the order, to destroy the
dependants before the dependencies.

This fixes the crash, that can be reproduced with the following steps:
```
frr# conf term file-lock
frr(config)# affinity-map map bit-position 10
frr(config)# interface test
frr(config-if)# link-params
frr(config-link-params)# affinity map
frr(config-link-params)# exit
frr(config-if)# exit
frr(config)# mgmt commit apply
frr(config)# no affinity-map map
frr(config)# interface test
frr(config-if)# link-params
frr(config-link-params)# no affinity map
frr(config-link-params)# exit
frr(config-if)# exit
frr(config)# mgmt commit apply
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>