crypto: ctr - convert to skcipher API

Convert the CTR template from the deprecated "blkcipher" API to the
"skcipher" API, taking advantage of skcipher_alloc_instance_simple() to
simplify it considerably.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>