ax25: fix a use-after-free in ax25_fillin_cb()

ax25: fix a use-after-free in ax25_fillin_cb()

There are multiple issues here:

1. After freeing dev->ax25_ptr, we need to set it to NULL otherwise
   we may use a dangling pointer.

2. There is a race between ax25_setsockopt() and device notifier as
   reported by syzbot. Close it by holding RTNL lock.

3. We need to test if dev->ax25_ptr is NULL before using it.

Reported-and-tested-by: [email protected]
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>