Clean up providing key via old API

- Use FSCRYPT_MAX_KEY_SIZE from <linux/fscrypt.h> instead of manually
  defining FS_AES_256_XTS_KEY_SIZE.  These have the same numeric value
  (64), but the former is supposed to be used, and AES-256-XTS isn't
  necessarily the encryption algorithm that is being used anyway.

- Use the new name FSCRYPT_KEY_DESCRIPTOR_SIZE instead of the old name
  FS_KEY_DESCRIPTOR_SIZE.  These have the same numeric value (8).

- Don't try to handle sizeof(fscrypt_key::raw) > FSCRYPT_MAX_KEY_SIZE,
  as this simply isn't the case.

- Set fscrypt_key::mode to 0 rather than FS_ENCRYPTION_MODE_AES_256_XTS.
  This field has always been ignored by the kernel, and AES-256-XTS
  isn't necessarily the encryption algorithm that is being used anyway.

- Initialize the fields of fscrypt_key in order.

This is a cleanup only.

Test: booted hikey with fileencryption=aes-256-xts and a kernel that
      doesn't support the new fscrypt ioctls.
Bug: none
Change-Id: Ie2a7e9240aa479dfab2765c11db8a7124d20c643