crypto: ahash - make struct ahash_instance be the full size

Define struct ahash_instance in a way analogous to struct
skcipher_instance, struct aead_instance, and struct akcipher_instance,
where the struct is defined to include both the algorithm structure at
the beginning and the additional crypto_instance fields at the end.

This is needed to allow allocating ahash instances directly using
kzalloc(sizeof(*inst) + sizeof(*ictx), ...) in the same way as skcipher,
aead, and akcipher instances.  In turn, that's needed to make spawns be
initialized in a consistent way everywhere.

Also take advantage of the addition of the base instance to struct
ahash_instance by simplifying the ahash_crypto_instance() and
ahash_instance() functions.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>