ubifs: use crypto_shash_tfm_digest() in ubifs_hmac_wkm()

Simplify ubifs_hmac_wkm() by using crypto_shash_tfm_digest() instead of
an alloc+init+update+final sequence.  This should also improve
performance.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/auth.c

@@ -507,28 +507,13 @@ int __ubifs_shash_copy_state(const struct ubifs_info *c, struct shash_desc *src,
  */
 int ubifs_hmac_wkm(struct ubifs_info *c, u8 *hmac)
 {
-	SHASH_DESC_ON_STACK(shash, c->hmac_tfm);
-	int err;
 	const char well_known_message[] = "UBIFS";
 
 	if (!ubifs_authenticated(c))
 		return 0;
 
-	shash->tfm = c->hmac_tfm;
-
-	err = crypto_shash_init(shash);
-	if (err)
-		return err;
-
-	err = crypto_shash_update(shash, well_known_message,
-				  sizeof(well_known_message) - 1);
-	if (err < 0)
-		return err;
-
-	err = crypto_shash_final(shash, hmac);
-	if (err)
-		return err;
-	return 0;
+	return crypto_shash_tfm_digest(c->hmac_tfm, well_known_message,
+				       sizeof(well_known_message) - 1, hmac);
 }
 
 /*