crypto: acomp - Add ACOMP_REQUEST_CLONE

Add a new helper ACOMP_REQUEST_CLONE that will transform a stack
request into a dynamically allocated one if possible, and otherwise
switch it over to the sycnrhonous fallback transform.  The intended
usage is:

	ACOMP_STACK_ON_REQUEST(req, tfm);

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

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

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