NFSD: add cb opcode to WARN_ONCE on failed callback

It helps to know what kind of callback happened that triggered the
WARN_ONCE in nfsd4_cb_done() function in diagnosing what can set
an uncommon state where both cb_status and tk_status are set at
the same time.

Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: Olga Kornievskaia

fs/nfsd/nfs4callback.c

@@ -1396,8 +1396,9 @@ static void nfsd4_cb_done(struct rpc_task *task, void *calldata)
 		return;
 
 	if (cb->cb_status) {
-		WARN_ONCE(task->tk_status, "cb_status=%d tk_status=%d",
-			  cb->cb_status, task->tk_status);
+		WARN_ONCE(task->tk_status,
+			  "cb_status=%d tk_status=%d cb_opcode=%d",
+			  cb->cb_status, task->tk_status, cb->cb_ops->opcode);
 		task->tk_status = cb->cb_status;
 	}