crypto: shash - Add __crypto_shash_cast

This patch adds __crypto_shash_cast which turns a crypto_tfm
into crypto_shash.  It's analogous to the other __crypto_*_cast
functions.

It hasn't been needed until now since no existing shash algorithms
have had an init function.

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