logd: report logging memory overhead

On 64 bit system, calculates to roughly 80 bytes of metadata and
list overhead for each entry.

In unit test example, we report 3388987 bytes of logging data and
overhead total, showmap reports 4652K of dirty data. We still want
to account for the remainder (fragmentation, other sources of
internal allocations etc).

Test: see values and check math
Bug: 31942525
Change-Id: I75f3162ce691faf1ae5a5dec18939fea535ede7e