netconsole: clean up deactivated targets dropped before the cleanup worker

drop_netconsole_target() downgrades a STATE_DEACTIVATED target to
STATE_DISABLED and then only calls netpoll_cleanup() when the target is
STATE_ENABLED. A target becomes STATE_DEACTIVATED when its underlying
interface is unregistered: netconsole_netdev_event() moves it to
target_cleanup_list, and netconsole_process_cleanups_core() is expected
to run do_netpoll_cleanup() on it.

Now that drop_netconsole_target() takes target_cleanup_list_lock around
the unlink, a configfs removal racing with NETDEV_UNREGISTER can pull the
target off target_cleanup_list before the cleanup worker processes it.
The notifier drops the lock before calling
netconsole_process_cleanups_core(), so the worker then iterates a list
that no longer contains the target and never runs do_netpoll_cleanup() on
it. Because drop_netconsole_target() has already rewritten the state to
STATE_DISABLED, its own STATE_ENABLED check is false and netpoll_cleanup()
is skipped too. The net_device reference taken by netpoll_setup() is then
leaked and unregister_netdevice() hangs forever in netdev_wait_allrefs().

Capture whether the target still owns a netpoll before the state is
downgraded and clean it up for both STATE_ENABLED and STATE_DEACTIVATED
targets. netpoll_cleanup() is idempotent -- it skips when np->dev is
already NULL -- so it is safe even when the cleanup worker won the race
and already tore the netpoll down.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260604-netcons_fix_before_move-v3-4-ab055b3a6aa5@debian.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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