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