[CRYPTO]: Use CPU cycle counters in tcrypt

After using this facility for a while to test my changes to the
cipher crypt() layer, I realised that I should've listend to Dave
and made this thing use CPU cycle counters :) As it is it's too
jittery for me to feel safe about relying on the results.

So here is a patch to make it use CPU cycles by default but fall
back to jiffies if the user specifies a non-zero sec value.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>