crypto: xcbc - Fix shash conversion

Although xcbc was converted to shash, it didn't obey the new
requirement that all hash state must be stored in the descriptor
rather than the transform.

This patch fixes this issue and also optimises away the rekeying
by precomputing K2 and K3 within setkey.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>