Returns an `rdf_crypto.Hash` instance for given AFF4 file descriptor.

def GetFileHashEntry(fd):
  """Returns an `rdf_crypto.Hash` instance for given AFF4 file descriptor."""
  # Hash file store is not migrated to RELDB just yet, hence the first check.
  if (not fd.urn.Path().startswith("/files/hash/generic") and
      data_store.RelationalDBEnabled()):
    return GetUrnHashEntry(fd.urn)
  else:
    return fd.Get(fd.Schema.HASH)