crypto: shash - Add shash_instance

This patch adds shash_instance and the associated alloc/free
functions.  This is meant to be an instance that with a shash
algorithm under it.  Note that the instance itself doesn't have
to be shash.

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

diff --cc crypto/shash.c
Simple merge

diff --cc include/crypto/internal/hash.h
Simple merge