This patch prepares the verification code for adding
a new verification step for the file storage paths:
- It moves a couple of file storage helper functions from
  bdev to filestorage (since they make more sense there
  and bdev is too big anyway).
- They rename constants and functions related to the
  verification step where the allowed file paths are
  checked agains forbidden paths to a more expressive name,
  because otherwise they would be confused to be related
  to the verification of the file storage paths against
  the allowed file storage paths.
- Use the cluster objects helper functions to check
  if file storage is inabled instead of using the utils
  function directly, because it simplifies the code.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>