Rename qcrypto_block_*crypt_helper to qcrypto_block_cipher_*crypt_helper,
as it's not about QCryptoBlock. This is needed to introduce
qcrypto_block_*crypt_helper in the next commit, which will have
QCryptoBlock pointer and than will be able to use additional fields of
it, which in turn will be used to implement thread-safe QCryptoBlock
operations.