crypto: ctr - add helper for performing a CTR encryption walk

Add a static inline helper modeled after crypto_cbc_encrypt_walk()
that can be reused for SIMD algorithms that need to implement a
non-SIMD fallback for performing CTR encryption.

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