crypto: tcrypt - Fix memory leaks/crashes in multibuffer hash speed test

This patch resolves a number of issues with the mb speed test
function:

* The tfm is never freed.
* Memory is allocated even when we're not using mb.
* When an error occurs we don't wait for completion for other requests.
* When an error occurs during allocation we may leak memory.
* The test function ignores plen but still runs for plen != blen.
* The backlog flag is incorrectly used (may crash).

This patch tries to resolve all these issues as well as making
the code consistent with the existing hash speed testing function.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>