calculating free size of internal storage from '/data' and not '/data/debian'

When lildebi is installed first time on a device, then /data/debian folder is not present. So when
the install screen (of installing in internal storage) tries to calculate the free size of internal storage
from /data/debian then an exception is generated as /data/debian is not present.
So instead of calculating free size from /data/debian, we will now calculate it from /data