crypto: hash - Add HASH_REQUEST_ON_STACK

Allow any ahash to be used with a stack request, with optional
dynamic allocation when async is needed.  The intended usage is:

	HASH_REQUEST_ON_STACK(req, tfm);

	...
	err = crypto_ahash_digest(req);
	/* The request cannot complete synchronously. */
	if (err == -EAGAIN) {
		/* This will not fail. */
		req = HASH_REQUEST_CLONE(req, gfp);

		/* Redo operation. */
		err = crypto_ahash_digest(req);
	}

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