crypto: shash - make struct shash_instance be the full size

Define struct shash_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 shash 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
shash_instance by simplifying the shash_crypto_instance() and
shash_instance() functions.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/internal/hash.h

@@ -34,7 +34,13 @@ struct ahash_instance {
 };
 
 struct shash_instance {
+	union {
+		struct {
+			char head[offsetof(struct shash_alg, base)];
+			struct crypto_instance base;
+		} s;
 		struct shash_alg alg;
+	};
 };
 
 struct crypto_ahash_spawn {
@@ -210,14 +216,13 @@ static inline void *crypto_shash_ctx(struct crypto_shash *tfm)
 static inline struct crypto_instance *shash_crypto_instance(
 	struct shash_instance *inst)
 {
-	return container_of(&inst->alg.base, struct crypto_instance, alg);
+	return &inst->s.base;
 }
 
 static inline struct shash_instance *shash_instance(
 	struct crypto_instance *inst)
 {
-	return container_of(__crypto_shash_alg(&inst->alg),
-			    struct shash_instance, alg);
+	return container_of(inst, struct shash_instance, s.base);
 }
 
 static inline struct shash_instance *shash_alg_instance(