Update csi-debug.md

Author: andyzhangx

docs/csi-debug.md

@@ -27,3 +27,9 @@ csi-smb-node-dr4s4                        3/3     Running   0          7m4s    1
 ```console
 $ kubectl logs csi-smb-node-cvgbs -c smb -n kube-system > csi-smb-node.log
 ```
+
+### troubleshooting connection failure on agent node
+```console
+mkdir /tmp/test
+sudo mount -v -t cifs //smb-server/fileshare /tmp/test -o ...
+```