SUNRPC: add side channel to use non-generic cred for rpc call.

The credential passed in rpc_message.rpc_cred is always a
generic credential except in one instance.
When gss_destroying_context() calls rpc_call_null(), it passes
a specific credential that it needs to destroy.
In this case the RPC acts *on* the credential rather than
being authorized by it.

This special case deserves explicit support and providing that will
mean that rpc_message.rpc_cred is *always* generic, allowing
some optimizations.

So add "tk_op_cred" to rpc_task and "rpc_op_cred" to the setup data.
Use this to pass the cred down from rpc_call_null(), and have
rpcauth_bindcred() notice it and bind it in place.

Credit to kernel test robot <fengguang.wu@intel.com> for finding
a bug in earlier version of this patch.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 26bea23..4cb697c 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -2534,12 +2534,12 @@ struct rpc_task *rpc_call_null_helper(struct rpc_clnt *clnt,
 {
 	struct rpc_message msg = {
 		.rpc_proc = &rpcproc_null,
-		.rpc_cred = cred,
 	};
 	struct rpc_task_setup task_setup_data = {
 		.rpc_client = clnt,
 		.rpc_xprt = xprt,
 		.rpc_message = &msg,
+		.rpc_op_cred = cred,
 		.callback_ops = (ops != NULL) ? ops : &rpc_default_ops,
 		.callback_data = data,
 		.flags = flags,