system.find_mount_point() uses os.path.ismount(), which can treat Btrfs subvolume roots as mount points even when findmnt does not list those roots as separate targets. When a Wine prefix was placed below such a subvolume, get_fs_type_for_path() looked for an exact findmnt target, found none, and returned None. Wine prefix creation then failed with: "Can't create the prefix on a file system that does not support Linux symbolic links."
Instead of matching a single mount point by exact path, choose the deepest findmnt target that contains the requested path. This resolves the backing filesystem for Btrfs subvolume paths and preserves the existing fuseblk type lookup.
