export force (-f --force) as unmount-option.

the following works now:
$uuid is a LUKS device containing a encrypted partition

udiskie-mount /dev/disk/by-uuid/$uuid
udiskie-umount -f /dev/disk/by-uuid/$uuid

this recursively umounts the encrypted partition and locks the LUKS device
afterwards. Before that udiskie-umount on the _device_ containing the encrypted
partition would fail.