RDMA/ipoib: Allow user space differentiate between valid dev_port

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: f6350da41dc7 ("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>