small nit: fix unmeaningful log
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 <14976e959b2860fb06a3c1edbdb1945a4e11b8b5@huawei.com>
(cherry picked from commit af8359562c9561afad0a05e66386588d17788804)
Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

joaofnfernandes/docker.github.io,johnstep/docker.github.io,joeuo/docker.github.io,rillig/docker.github.io,LuisBosquez/docker.github.io,joaofnfernandes/docker.github.io,jzwlqx/denverdino.github.io,LuisBosquez/docker.github.io,johnstep/docker.github.io,anweiss/docker.github.io,denverdino/denverdino.github.io,denverdino/docker.github.io,denverdino/denverdino.github.io,jzwlqx/denverdino.github.io,docker-zh/docker.github.io,sanscontext/docker.github.io,docker/docker.github.io,phiroict/docker,alexisbellido/docker.github.io,thaJeztah/docker.github.io,BSWANG/denverdino.github.io,JimGalasyn/docker.github.io,jzwlqx/denverdino.github.io,denverdino/denverdino.github.io,shubheksha/docker.github.io,docker/docker.github.io,joaofnfernandes/docker.github.io,JimGalasyn/docker.github.io,medallia/docker,stainboy/docker,anweiss/docker.github.io,menglingwei/denverdino.github.io,LuisBosquez/docker.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,johnstep/docker.github.io,docker-zh/docker.github.io,docker-zh/docker.github.io,gdevillele/docker.github.io,londoncalling/docker.github.io,medallia/docker,gdevillele/docker.github.io,denverdino/docker.github.io,shin-/docker.github.io,joeuo/docker.github.io,troy0820/docker.github.io,sanscontext/docker.github.io,shin-/docker.github.io,bdwill/docker.github.io,denverdino/docker.github.io,bdwill/docker.github.io,sanscontext/docker.github.io,thaJeztah/docker.github.io,BSWANG/denverdino.github.io,danix800/docker.github.io,bdwill/docker.github.io,JimGalasyn/docker.github.io,shin-/docker.github.io,docker-zh/docker.github.io,thaJeztah/docker.github.io,menglingwei/denverdino.github.io,phiroict/docker,rillig/docker.github.io,BSWANG/denverdino.github.io,thaJeztah/docker.github.io,phiroict/docker,joeuo/docker.github.io,aduermael/docker.github.io,anweiss/docker.github.io,gdevillele/docker.github.io,gdevillele/docker.github.io,bdwill/docker.github.io,aduermael/docker.github.io,denverdino/denverdino.github.io,LuisBosquez/docker.github.io,troy0820/docker.github.io,alexisbellido/docker.github.io,BSWANG/denverdino.github.io,Shopify/docker,rillig/docker.github.io,gdevillele/docker.github.io,jzwlqx/denverdino.github.io,denverdino/docker.github.io,menglingwei/denverdino.github.io,sanscontext/docker.github.io,joaofnfernandes/docker.github.io,aduermael/docker.github.io,joaofnfernandes/docker.github.io,JimGalasyn/docker.github.io,shubheksha/docker.github.io,medallia/docker,docker-zh/docker.github.io,londoncalling/docker.github.io,sanscontext/docker.github.io,londoncalling/docker.github.io,stainboy/docker,joeuo/docker.github.io,LuisBosquez/docker.github.io,BSWANG/denverdino.github.io,joeuo/docker.github.io,shubheksha/docker.github.io,danix800/docker.github.io,Shopify/docker,stainboy/docker,thaJeztah/docker.github.io,danix800/docker.github.io,troy0820/docker.github.io,troy0820/docker.github.io,alexisbellido/docker.github.io,alexisbellido/docker.github.io,londoncalling/docker.github.io,shubheksha/docker.github.io,bdwill/docker.github.io,docker/docker.github.io,phiroict/docker,shin-/docker.github.io,rillig/docker.github.io,danix800/docker.github.io,medallia/docker,jzwlqx/denverdino.github.io,johnstep/docker.github.io,docker/docker.github.io,anweiss/docker.github.io,aduermael/docker.github.io,londoncalling/docker.github.io,menglingwei/denverdino.github.io,denverdino/denverdino.github.io,johnstep/docker.github.io,denverdino/docker.github.io,phiroict/docker,alexisbellido/docker.github.io,shubheksha/docker.github.io,menglingwei/denverdino.github.io,shin-/docker.github.io,Shopify/docker,anweiss/docker.github.io