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/sched.c b/net/sunrpc/sched.c
index 57ca5be..c9f6503 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -997,6 +997,8 @@ static void rpc_init_task(struct rpc_task *task, const struct rpc_task_setup *ta
 
 	task->tk_xprt = xprt_get(task_setup_data->rpc_xprt);
 
+	task->tk_op_cred = get_rpccred(task_setup_data->rpc_op_cred);
+
 	if (task->tk_ops->rpc_call_prepare != NULL)
 		task->tk_action = rpc_prepare_task;
 
@@ -1054,6 +1056,7 @@ static void rpc_free_task(struct rpc_task *task)
 {
 	unsigned short tk_flags = task->tk_flags;
 
+	put_rpccred(task->tk_op_cred);
 	rpc_release_calldata(task->tk_ops, task->tk_calldata);
 
 	if (tk_flags & RPC_TASK_DYNAMIC) {