fix layer size computation: handle hard links

This change makes docker compute layer size correctly.

The old code isn't taking hard links into account. Layers could
seem like they're up to 1-1.5x larger than they really were.
fix container size computation: handle hard links

This change makes docker compute container size correctly.

The old code isn't taking hard links into account. Containers could
seem like they're up to 1-1.5x larger than they really were.