ax25: fix UAF bug in ax25_send_control()

There are UAF bugs in ax25_send_control(), when we call ax25_release()
to deallocate ax25_dev. The possible race condition is shown below:

      (Thread 1)              |     (Thread 2)
ax25_dev_device_up() //(1)    |
                              | ax25_kill_by_device()
ax25_bind()          //(2)    |
ax25_connect()                | ...
 ax25->state = AX25_STATE_1   |
 ...                          | ax25_dev_device_down() //(3)

      (Thread 3)
ax25_release()                |
 ax25_dev_put()  //(4) FREE   |
 case AX25_STATE_1:           |
  ax25_send_control()         |
   alloc_skb()       //USE    |

The refcount of ax25_dev increases in position (1) and (2), and
decreases in position (3) and (4). The ax25_dev will be freed
before dereference sites in ax25_send_control().

The following is part of the report:

[  102.297448] BUG: KASAN: use-after-free in ax25_send_control+0x33/0x210
[  102.297448] Read of size 8 at addr ffff888009e6e408 by task ax25_close/602
[  102.297448] Call Trace:
[  102.303751]  ax25_send_control+0x33/0x210
[  102.303751]  ax25_release+0x356/0x450
[  102.305431]  __sock_release+0x6d/0x120
[  102.305431]  sock_close+0xf/0x20
[  102.305431]  __fput+0x11f/0x420
[  102.305431]  task_work_run+0x86/0xd0
[  102.307130]  get_signal+0x1075/0x1220
[  102.308253]  arch_do_signal_or_restart+0x1df/0xc00
[  102.308253]  exit_to_user_mode_prepare+0x150/0x1e0
[  102.308253]  syscall_exit_to_user_mode+0x19/0x50
[  102.308253]  do_syscall_64+0x48/0x90
[  102.308253]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  102.308253] RIP: 0033:0x405ae7

This patch defers the free operation of ax25_dev and net_device after
all corresponding dereference sites in ax25_release() to avoid UAF.

Fixes: 9fd75b66 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>