crypto: hash - Simplify the ahash_finup implementation

The ahash_def_finup() can make use of the request save/restore functions,
thus make it so. This simplifies the code a little and unifies the code
paths.

Note that the same remark about free()ing the req->priv applies here, the
req->priv can only be free()'d after the original request was restored.

Finally, squash a bug in the invocation of completion in the ASYNC path.
In both ahash_def_finup_done{1,2}, the function areq->base.complete(X, err);
was called with X=areq->base.data . This is incorrect , as X=&areq->base
is the correct value. By analysis of the data structures, we see the areq is
of type 'struct ahash_request' , areq->base is of type 'struct crypto_async_request'
and areq->base.completion is of type crypto_completion_t, which is defined in
include/linux/crypto.h as:

  typedef void (*crypto_completion_t)(struct crypto_async_request *req, int err);

This is one lead that the X should be &areq->base . Next up, we can inspect
other code whi...