dissect-image: fix volatile images

This makes sure nspawn's --volatile=yes switch works again: there we
have a read-only image that is overmounted by a tmpfs (with the
exception of /usr). This we need to mkdir all mount points even though
the image is read-only.

Hence, let's drop the optimizatio of avoiding mkdir() on images that are
read-only, it's wrong and misleading here, since the image itself might
be read-only but our mounts are not.