[CRYPTO] api: Add type-safe spawns

This patch allows spawns of specific types (e.g., cipher) to be allocated.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/cbc.c b/crypto/cbc.c
index f5542b4..136fea7 100644
--- a/crypto/cbc.c
+++ b/crypto/cbc.c
@@ -243,6 +243,7 @@
 	struct crypto_instance *inst = (void *)tfm->__crt_alg;
 	struct crypto_spawn *spawn = crypto_instance_ctx(inst);
 	struct crypto_cbc_ctx *ctx = crypto_tfm_ctx(tfm);
+	struct crypto_cipher *cipher;
 
 	switch (crypto_tfm_alg_blocksize(tfm)) {
 	case 8:
@@ -260,11 +261,11 @@
 			ctx->xor = xor_quad;
 	}
 
-	tfm = crypto_spawn_tfm(spawn);
-	if (IS_ERR(tfm))
-		return PTR_ERR(tfm);
+	cipher = crypto_spawn_cipher(spawn);
+	if (IS_ERR(cipher))
+		return PTR_ERR(cipher);
 
-	ctx->child = crypto_cipher_cast(tfm);
+	ctx->child = cipher;
 	return 0;
 }