ax25: Fix reference count leak issue of net_device

In the Linux kernel, the following vulnerability has been resolved:

ax25: Fix reference count leak issue of net_device

There is a reference count leak issue of the object "net_device" in

ax25_dev_device_down(). When the ax25 device is shutting down, the

ax25_dev_device_down() drops the reference count of net_device one

or zero times depending on if we goto unlock_put or not, which will

cause memory leak.

In order to solve the above issue, decrease the reference count of

net_device after dev->ax25_ptr is set to null.