bug#50432: DF shows incorrect disk usage which include disk usage of a mounted path

[Paco Xu]

Hi,

I am not quite familiar with the bug report process of `DF` or os.

The Problem

-      DF shows incorrect disk usage which include disk usage of a mounted path

The Scenario
We run a registry with docker and the data dir is mounted from NFS or local 
disk. The docker is using device-mapper.

DF result & du result
When registry is busy pulling images, there is no big files in registry 
container, but the `df -h` shows 7.8 G as below(Sometime 9G the below 
screenshot 8.5G)
```
[root@10-23-15-8 ~]# df -h | grep  
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
/dev/dm-10                10G  7.8G  2.3G   78% 
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
```
The /var/lib/registry/docker is mounted
```
            {
                "Type": "bind",
                "Source": "/var/local/dce/registry",
                "Destination": "/var/lib/registry",
                "Mode": "rw",
                "RW": true,
                "Propagation": "rprivate"
            }
```
[文本  描述已自动生成]

And at the time, I run `du`.
```
[root@10-23-15-8 ~]# du -h 
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
 | tail -n 1
478M       
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
```
The du details shows that `df -h` is not correct.

The expected behavior
When it became normal, the disk usage is about 400~500M.
```
[root@10-23-15-8 ~]# df -h | grep  
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
/dev/dm-10                10G  478M  9.5G    6% 
/var/lib/docker/devicemapper/mnt/a0ba326c248009e14e9650c84818fdb45af904338bdc90e7d786a584f63ed47e
```
No matter what happened, the `df -h` should not take `mounted path 
./var/lib/registry/docker` into account.

Further Information
I searched in stackoverflow and other place and only find some issues about `df 
and du show different` problem. I am not sure which is my case. Thanks.

Best regards,
Paco
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