Set the "expire" flag on the filesystem.
const UNMOUNT_EXPIRE: UnmountFlags = UnmountFlags(MNT_EXPIRE);
The "expire" flag is automatically unset when the filesystem is accessed again. If an unmount operation with this flag set is used and the "expire" flag is already set on the filesystem, then the device is unmounted.
umount(2) and MNT_EXPIRE therein