crypto: tcrypt - use multi buf for ahash mb test

The multi buffer ahash speed test was allocating multiple
buffers for use with the multiple outstanding requests
it was starting but never actually using them (except
to free them), instead using a different single statically
allocated buffer for all requests.

Fix this by actually using the allocated buffers for the test.

It is noted that it may seem tempting to instead remove the
allocation and free of the multiple buffers and leave things as
they are since this is a hash test where the input is read
only. However, after consideration I believe that multiple
buffers better reflect real life scenario with regard
to data cache and TLB behaviours etc.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>