net: Hold rtnl_net_lock() in (un)?register_netdevice_notifier_net().

(un)?register_netdevice_notifier_net() hold RTNL before triggering the
notifier for all netdev in the netns.

Let's convert the RTNL to rtnl_net_lock().

Note that the per-netns netdev notifier is protected by per-netns RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250106070751.63146-3-kuniyu@amazon.com


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