Add Adiantum encryption mode and remove unused modes
Add Adiantum encryption mode and remove unused modes

Update the list of available encryption modes:

- Add the Adiantum encryption mode (supported since Linux v5.0).

- Remove AES-256-GCM and AES-256-CBC, since the kernel has never
  actually suported those, and their constants have been re-reserved.

- Stop using the special value "FSCRYPT_MODE_INVALID"; it's not needed.

google/fscryptctl,google/fscryptctl