dm crypt: use crypto_shash_digest() in crypt_iv_tcw_whitening()

Simplify crypt_iv_tcw_whitening() by using crypto_shash_digest() instead
of an init+update+final sequence.  This should also improve performance.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-crypt.c

@@ -652,13 +652,7 @@ static int crypt_iv_tcw_whitening(struct crypt_config *cc,
 	/* calculate crc32 for every 32bit part and xor it */
 	desc->tfm = tcw->crc32_tfm;
 	for (i = 0; i < 4; i++) {
-		r = crypto_shash_init(desc);
-		if (r)
-			goto out;
-		r = crypto_shash_update(desc, &buf[i * 4], 4);
-		if (r)
-			goto out;
-		r = crypto_shash_final(desc, &buf[i * 4]);
+		r = crypto_shash_digest(desc, &buf[i * 4], 4, &buf[i * 4]);
 		if (r)
 			goto out;
 	}