cryptfs: also unmount /data/data before encrypting

galaxysmtd mounts /data/data on top on /data when unencrypted for
performance reason, so when encrypting, we need to unmount that
first before we can unmount /data.

On other devices, umount on a non-mounted folder will silently
succeed.

Change-Id: I07a73534fb3e253b702a4be6f976594644795e36