fscrypt: use crypto_shash_tfm_digest()

Instead of manually allocating a 'struct shash_desc' on the stack and
calling crypto_shash_digest(), switch to using the new helper function
crypto_shash_tfm_digest() which does this for us.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
index efb95bd..0cba792 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -44,17 +44,13 @@ static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
 			unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
 {
 	static const u8 default_salt[HKDF_HASHLEN];
-	SHASH_DESC_ON_STACK(desc, hmac_tfm);
 	int err;
 
 	err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
 	if (err)
 		return err;
 
-	desc->tfm = hmac_tfm;
-	err = crypto_shash_digest(desc, ikm, ikmlen, prk);
-	shash_desc_zero(desc);
-	return err;
+	return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
 }
 
 /*