Platform's add/remove operations accept a "network" argument.
Kernel requires that the host part (based on plen) is all zero.
For NetworkManager we are more resilient to user configuration.

Cleanup the input argument already before calling _nl_msg_new_route().
Note that we use the same "network" argument to construct a obj_id
instance and to find the route in the cache (do_add_addrroute()).
Without cleaning the host part, the added object cannot be found
and the add-route command seemingly fails.