crypto: arm64/aes-cts-cbc - factor out CBC en/decryption of a walk

The plain CBC driver and the CTS one share some code that iterates over
a scatterwalk and invokes the CBC asm code to do the processing. The
upcoming ESSIV/CBC mode will clone that pattern for the third time, so
let's factor it out first.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>