common/encrypt: support verifying ciphertext of IV_INO_LBLK_64 policies

common/encrypt: support verifying ciphertext of IV_INO_LBLK_64 policies

Update _verify_ciphertext_for_encryption_policy() to support encryption
policies with the IV_INO_LBLK_64 flag set.

This flag modifies the encryption to include the inode number in the IVs
and to use a key derived from the tuple [master_key, fs_uuid, mode_num].
Since the file nonce is *not* included in this key derivation, multiple
files can use the same key.

This flag is supported by v2 encryption policies only -- not by v1.

Signed-off-by: Eric Biggers <ebiggers@google.com>