ax25: Fix reference count leak issues of ax25_dev

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

ax25: Fix reference count leak issues of ax25_dev

The ax25_addr_ax25dev() and ax25_dev_device_down() exist a reference

count leak issue of the object "ax25_dev".

Memory leak issue in ax25_addr_ax25dev():

The reference count of the object "ax25_dev" can be increased multiple

times in ax25_addr_ax25dev(). This will cause a memory leak.

Memory leak issues in ax25_dev_device_down():

The reference count of ax25_dev is set to 1 in ax25_dev_device_up() and

then increase the reference count when ax25_dev is added to ax25_dev_list.

As a result, the reference count of ax25_dev is 2. But when the device is

shutting down. The ax25_dev_device_down() drops the reference count once

or twice depending on if we goto unlock_put or not, which will cause

memory leak.

As for the issue of ax25_addr_ax25dev(), it is impossible for one pointer

to be on a list twice. So add a break in ax25_addr_ax25dev(). As for the

issue of ax25_dev_device_down(), increase the reference count of ax25_dev

once in ax25_dev_device_up() and decrease the reference count of ax25_dev

after it is removed from the ax25_dev_list.