tar: fixing extracting files that are dirs 

Fix https://github.com/coreos/rocket/issues/250

The dirs are created using [0755](https://github.com/mcuadros/rocket/compare/coreos:master...master#diff-6b4b95c3fb2a650ac8b9a0c5eabdb8b3R13) as default mode, following the same behaviour from the BSD tar.

Urgh, that sucks. Since rocket is (currently) only targeted at Linux I think principle of least surprise says we should go with the GNU approach, mind doing that?