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.

@mcuadros my only concern here is that entries in a tar are not necessarily lexically ordered, so what if we encounter one of these parent directories later in the tar (e.g. with different permissions)?

I haven't dug into the GNU tar code to see how they handle this..