[th/l3cd-empty] l3cfg: make nm_l3_config_data_cmp_full() thread-safe

Let's not have unexpected, non-thread-safe functions somewhere deep down.

NML3ConfigData -- as a data structure -- is not thread-safe, nor aims it to be. However, our code(!) should be thread-safe. That means, it should be possible to call our code on separate data from multiple threads. Violating that is a code smell and a foot gun.

This basically means that code should not access global data (unless thread-local) or that the access to global-data needs to be synchronized.