crypto/builtin: Split and simplify AES_encrypt_cbc

Split into encrypt/decrypt functions, dropping the "enc" argument.
Now that the function is private to this file, we know that "len"
is a multiple of AES_BLOCK_SIZE.  So drop the odd block size code.

Name the functions do_aes_*crypt_cbc to match the *_ecb functions.
Reorder and re-type the arguments to match as well.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>