crypto: talitos - zeroize the descriptor with memset()

This patch zeroize the descriptor at allocation using memset().
This has two advantages:
- It reduces the number of places where data has to be set to 0
- It avoids reading memory and loading the cache with data that
will be entirely replaced.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>