crypto: skcipher - Add helper to zero stack request

As the size of an skcipher_request is variable, it's awkward to
zero it explicitly.  This patch adds a helper to do that which
should be used when it is created on the stack.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/skcipher.h

@@ -445,6 +445,13 @@ static inline void skcipher_request_free(struct skcipher_request *req)
 	kzfree(req);
 }
 
+static inline void skcipher_request_zero(struct skcipher_request *req)
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+
+	memzero_explicit(req, sizeof(*req) + crypto_skcipher_reqsize(tfm));
+}
+
 /**
  * skcipher_request_set_callback() - set asynchronous callback function
  * @req: request handle