rtnetlink: Use register_pernet_subsys() in rtnl_net_debug_init().

rtnl_net_debug_init() registers rtnl_net_debug_net_ops by
register_pernet_device() but calls unregister_pernet_subsys()
in case register_netdevice_notifier() fails.

It corrupts pernet_list because first_device is updated in
register_pernet_device() but not unregister_pernet_subsys().

Let's fix it by calling register_pernet_subsys() instead.

The _subsys() one fits better for the use case because it keeps
the notifier alive until default_device_exit_net(), giving it
more chance to test NETDEV_UNREGISTER.

Fixes: 03fa5348 ("rtnetlink: Add ASSERT_RTNL_NET() placeholder for netdev notifier.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250401190716.70437-1-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>