This patch adds the frontend interface for asynchronous block ciphers.
In addition to the usual block cipher parameters, there is a callback
function pointer and a data pointer.  The callback will be invoked only
if the encrypt/decrypt handlers return -EINPROGRESS.  In other words,
if the return value of zero the completion handler (or the equivalent
code) needs to be invoked by the caller.

The request structure is allocated and freed by the caller.  Its size
is determined by calling crypto_ablkcipher_reqsize().  The helpers
ablkcipher_request_alloc/ablkcipher_request_free can be used to manage
the memory for a request.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>