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Containerized subpath
**What this PR does / why we need it**:
Containerized kubelet needs a different implementation of `PrepareSafeSubpath` than kubelet running directly on the host.
On the host we safely open the subpath and then bind-mount `/proc/<pidof kubelet>/fd/<descriptor of opened subpath>`.
With kubelet running in a container, `/proc/xxx/fd/yy` on the host contains path that works only inside the container, i.e. `/rootfs/path/to/subpath` and thus any bind-mount on the host fails.
Solution:
- safely open the subpath and gets its device ID and inode number
- blindly bind-mount the subpath to `/var/lib/kubelet/pods/<uid>/volume-subpaths/<name of container>/<id of mount>`. This is potentially unsafe, because user can change the subpath source to a link to a bad place (say `/run/docker.sock`) just before the bind-mount.
- get device ID and inode number of the destination. Typical users can't modify this file, as it lies on /var/lib/kubelet on the host.
- compare these device IDs and inode numbers.
**Which issue(s) this PR fixes**
Fixes#61456
**Special notes for your reviewer**:
The PR contains some refactoring of `doBindSubPath` to extract the common code. New `doNsEnterBindSubPath` is added for the nsenter related parts.
**Release note**:
```release-note
NONE
```
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