According to the Herbert Xu, client may not always call
crypto_ahash_final().

In the case of error in hash calculation resources will be
automatically cleaned up.

But if no hash calculation error happens and client will not call
crypto_ahash_final() at all, then internal buffer will not be freed,
and clocks will not be disabled.

This patch provides support for atomic crypto_ahash_update() call.
Clocks are now enabled and disabled per update request.

Data buffer is now allocated as a part of request context.
Client is obligated to free it with crypto_free_ahash().
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>