[Virtio-fs] [virtiofsd] MR merged: Fixes failed open mount point when using file handles with chroot

[virtiofs-bot at sinrega.org]

When using a mount point as a shared directory (e.g. bind mount),
using `--sandbox=chroot` and `--inode-file-handles=mandatory/prefer`,
virtiofsd fails to open the mount point, with the following error:

```
[2022-06-03T15:44:22Z ERROR virtiofsd::passthrough] Filesystem mounted on "" (mount ID: 744): Failed to open mount point "": No such file or directory (os rror 2)
[2022-06-03T15:44:22Z ERROR virtiofsd] Failed to create internal filesystem representation: Os { code: 2, kind: NotFound, message: "No such file or irectory" }
```

When using `--inode-file-handles=mandatory/prefer` checks whether the
root's (i.e. the shared directory) file handle is usable, calling
`check_working_file_handles()` from within `PassthroughFs::new()`.
To do this we need to determine the filesystem's mount point form
the mount_id in `/proc/self/mountinfo` and open it. But, when using
`--sandbox=chroot`,  `/proc/self/mountinfo` still contains the 
pre-chroot mount point paths. To open filesystem's mount point,
within 'get_mount_root()', we need to strip the shared directory as a
prefix from it.

However, when the shared directory is a mount point (e.g. when is bind
mounted) striping the shared directory from the filesystem's mount
point:

```rust
    if let Some(suffix) = p.strip_prefix(prefix) {
         Ok(suffix.into())
    } else {
         Ok("/".into())
    }
```
    
the `p.strip_prefix(prefix)` call returns `Some("")`, because the
shared directory and the filesystem's mount point are equal. And
that means that `get_mount_root()` instead of returning `Ok("\")` it
returns `Ok("")` that cannot be opened. A quick fix is to filter out
`""` to return `None`.

This is a temporal solution, a better solution, suggested by Vivek,
is to open `/proc/self`, call `chroot(shared_dir)`, and then open
`/proc/self/mountinfo` using the previous `/proc/self` file descriptor.
So, now the paths in `/proc/self/mountinfo` are relative to shared 
directory. This makes unnecessary the mountinfo_prefix logic.
Since this solution requires a wider change and needs to be tested
well to make sure we don't break any supported configuration, it will
be implemented in a future MR.
---
https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/124