crypto: tcrypt - allow setting num of bufs

For multiple buffers speed tests, the number of buffers, or
requests, used actually sets the level of parallelism a tfm
provider may utilize to hide latency. The existing number
(of 8) is good for some software based providers but not
enough for many HW providers with deep FIFOs.

Add a module parameter that allows setting the number of
multiple buffers/requests used, leaving the default at 8.

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