sunrpc: use kmemdup_nul() in gssp_stringify()

It is more efficient to use kmemdup_nul() if the size is known exactly
.

According to doc:
"Note: Use kmemdup_nul() instead if the size is known exactly."
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/gss_rpc_upcall.c

@@ -223,7 +223,7 @@ static int gssp_alloc_receive_pages(struct gssx_arg_accept_sec_context *arg)
 
 static char *gssp_stringify(struct xdr_netobj *netobj)
 {
-	return kstrndup(netobj->data, netobj->len, GFP_KERNEL);
+	return kmemdup_nul(netobj->data, netobj->len, GFP_KERNEL);
 }
 
 static void gssp_hostbased_service(char **principal)