block: cryptoloop: Remove VLA usage of skcipher

In the quest to remove all stack VLA usage from the kernel[1], this
replaces struct crypto_skcipher and SKCIPHER_REQUEST_ON_STACK() usage
with struct crypto_sync_skcipher and SYNC_SKCIPHER_REQUEST_ON_STACK(),
which uses a fixed stack size.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=[e-mail]

Cc: Jens Axboe <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

drivers/block/cryptoloop.c

···
 	char cms[LO_NAME_SIZE];			/* cipher-mode string */
 	char *mode;
 	char *cmsp = cms;			/* c-m string pointer */
-	struct crypto_skcipher *tfm;
+	struct crypto_sync_skcipher *tfm;
 
 	/* encryption breaks for non sector aligned offsets */
 
···
 	*cmsp++ = ')';
 	*cmsp = 0;
 
-	tfm = crypto_alloc_skcipher(cms, 0, CRYPTO_ALG_ASYNC);
+	tfm = crypto_alloc_sync_skcipher(cms, 0, 0);
 	if (IS_ERR(tfm))
 		return PTR_ERR(tfm);
 
-	err = crypto_skcipher_setkey(tfm, info->lo_encrypt_key,
-				     info->lo_encrypt_key_size);
-	
+	err = crypto_sync_skcipher_setkey(tfm, info->lo_encrypt_key,
+					  info->lo_encrypt_key_size);
+
 	if (err != 0)
 		goto out_free_tfm;
 
···
 	return 0;
 
  out_free_tfm:
-	crypto_free_skcipher(tfm);
+	crypto_free_sync_skcipher(tfm);
 
  out:
 	return err;
···
 		    struct page *loop_page, unsigned loop_off,
 		    int size, sector_t IV)
 {
-	struct crypto_skcipher *tfm = lo->key_data;
-	SKCIPHER_REQUEST_ON_STACK(req, tfm);
+	struct crypto_sync_skcipher *tfm = lo->key_data;
+	SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);
 	struct scatterlist sg_out;
 	struct scatterlist sg_in;
 
···
 	unsigned in_offs, out_offs;
 	int err;
 
-	skcipher_request_set_tfm(req, tfm);
+	skcipher_request_set_sync_tfm(req, tfm);
 	skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP,
 				      NULL, NULL);
 
···
 static int
 cryptoloop_release(struct loop_device *lo)
 {
-	struct crypto_skcipher *tfm = lo->key_data;
+	struct crypto_sync_skcipher *tfm = lo->key_data;
 	if (tfm != NULL) {
-		crypto_free_skcipher(tfm);
+		crypto_free_sync_skcipher(tfm);
 		lo->key_data = NULL;
 		return 0;
 	}