crypto: aead - Add aead_alg_instance

Now that type-safe init/exit functions exist, they often need
to access the underlying aead_instance.  So this patch adds the
helper aead_alg_instance to access aead_instance from a crypto_aead
object.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/internal/aead.h

@@ -52,6 +52,11 @@ static inline struct aead_instance *aead_instance(struct crypto_instance *inst)
 	return container_of(&inst->alg, struct aead_instance, alg.base);
 }
 
+static inline struct aead_instance *aead_alg_instance(struct crypto_aead *aead)
+{
+	return aead_instance(crypto_aead_alg_instance(aead));
+}
+
 static inline void *aead_instance_ctx(struct aead_instance *inst)
 {
 	return crypto_instance_ctx(aead_crypto_instance(inst));