Zhang Wei 5d53107934 small nit: fix unmeaningful log ...

Truncated dir name can't give any useful information, print whole dir
name will.

Bad debug log is like this:

```
DEBU[2449] aufs error unmounting /var/lib/doc: no such file or directory
```

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>
Upstream-commit: af8359562c9561afad0a05e66386588d17788804
Component: engine

2016-07-26 07:10:28 +08:00

..

engine

small nit: fix unmeaningful log

2016-07-26 07:10:28 +08:00