RDMA/ipoib: Allow user space differentiate between valid dev_port

Systemd triggers the following warning during IPoIB device load:

 mlx5_core 0000:00:0c.0 ib0: "systemd-udevd" wants to know my dev_id.
        Should it look at dev_port instead?
        See Documentation/ABI/testing/sysfs-class-net for more info.

This is caused due to user space attempt to differentiate old systems
without dev_port and new systems with dev_port. In case dev_port will be
zero, the systemd will try to read dev_id instead.

There is no need to print a warning in such case, because it is valid
situation and it is needed to ensure systemd compatibility with old
kernels.

Link: https://github.com/systemd/systemd/blob/master/src/udev/udev-builtin-net_id.c#L358
Cc: <stable@vger.kernel.org> # 4.19
Fixes: f6350da4 ("IB/ipoib: Log sysfs 'dev_id' accesses from userspace")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -2402,7 +2402,18 @@ static ssize_t dev_id_show(struct device *dev,
 {
 	struct net_device *ndev = to_net_dev(dev);
 
-	if (ndev->dev_id == ndev->dev_port)
+	/*
+	 * ndev->dev_port will be equal to 0 in old kernel prior to commit
+	 * 9b8b2a323008 ("IB/ipoib: Use dev_port to expose network interface
+	 * port numbers") Zero was chosen as special case for user space
+	 * applications to fallback and query dev_id to check if it has
+	 * different value or not.
+	 *
+	 * Don't print warning in such scenario.
+	 *
+	 * https://github.com/systemd/systemd/blob/master/src/udev/udev-builtin-net_id.c#L358
+	 */
+	if (ndev->dev_port && ndev->dev_id == ndev->dev_port)
 		netdev_info_once(ndev,
 			"\"%s\" wants to know my dev_id. Should it look at dev_port instead? See Documentation/ABI/testing/sysfs-class-net for more info.\n",
 			current->comm);