User reported netlink-related panic [kern/270813](https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270813) when unloading `if_ena` kernel module.
The crash happens when the swi-enqueued link state event is triggered. By the time of execution, the `ifp` in question and its resources are already freed, triggering the panic.
The following call chain leads to this:
ena_detach()
ether_ifdetach()
if_detach()
if_detach_internal() # here we drain taskqueue_swi from link events
ena_destroy_device()
if_link_state_change()
In order to fix the bug, this change proposes ignoring all link state events after ifnet was unlinked. The implementation assumes that the destruction happens in the single thread and avoid lock-based flag checking.