When overmem, clean enough memory when adding new ADB names/entries

When overmem, clean enough memory when adding new ADB names/entries

The purge_stale_names()/purge_stale_entries() is opportunistic even when
we are under memory pressure (overmem).  Split the opportunistic LRU
cleaning and overmem cleaning.  This makes the stale purging much
simpler as we don't have to try that hard and makes the overmem cleaning
always cleanup double the amount of the newly allocated ADB name/entry.