Function require_dir_contains_only_mounts 

fn require_dir_contains_only_mounts(
    parent_fd: &Dir,
    dir_name: &str,
) -> Result<()>

Require that a directory contains only mount points recursively. Returns Ok(()) if all entries in the directory tree are either:

• Mount points (on different filesystems)
• Directories that themselves contain only mount points (recursively)
• The lost+found directory

Returns an error if any non-mount entry is found.

This handles cases like /var containing /var/lib (not a mount) which contains /var/lib/containers (a mount point).