KEYS: Use memzero_explicit() for secret data

I don't think GCC has figured out how to optimize the memset() away, but
they might eventually so let's future proof this code a bit.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/keys/encrypted-keys/encrypted.c

@@ -985,7 +985,7 @@ static void encrypted_destroy(struct key *key)
 	if (!epayload)
 		return;
 
-	memset(epayload->decrypted_data, 0, epayload->decrypted_datalen);
+	memzero_explicit(epayload->decrypted_data, epayload->decrypted_datalen);
 	kfree(key->payload.data[0]);
 }