network: fix segfault in link_update_hardware_address()

network: fix segfault in link_update_hardware_address()

This fixes a bug introduced by fe321d457c118be5d1640b586135a69d1db9270a.

When we want to update a value (in the case of this commit, it is a hardware
address) which is used as a hashmap key, we need to do the following steps:
1. remove the old hashmap entry,
2. update the value,
3. create a new hashmap entry with the new value.