netconsole: close netdevice unregister window during target resume

netconsole: close netdevice unregister window during target resume

process_resume_target() removes the target from target_list before
calling resume_target() so that netpoll_setup() can run with interrupts
enabled, then re-adds it once setup completes. netpoll_setup() acquires a
net_device reference (netdev_hold()) and releases the RTNL before
returning.

While the target is off target_list and the RTNL is not held,
netconsole_netdev_event() cannot find it. If the egress device is
unregistered in that window, the NETDEV_UNREGISTER notifier walks
target_list, misses the resuming target, and never tears it down. The
target is then re-added in STATE_ENABLED still holding a reference to the
now-unregistered device, leaking it and hanging unregister_netdevice() in
netdev_wait_allrefs().

Re-check under RTNL before re-publishing the target: if the device left
NETREG_REGISTERED while we were off the list, run do_netpoll_cleanup() and
mark the target disabled. Taking the RTNL across the check and the
list_add() serialises against the NETDEV_UNREGISTER notifier, which also
runs under RTNL, so the device is either still registered (and the
notifier will find the re-added target later) or already unregistering
(and we drop the reference here). netdev_wait_allrefs() runs from
netdev_run_todo() outside the RTNL, so dropping the reference here cannot
deadlock against the pending unregister.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260604-netcons_fix_before_move-v3-5-ab055b3a6aa5@debian.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>