net: Hold __rtnl_net_lock() in (un)?register_netdevice_notifier().

(un)?register_netdevice_notifier() hold pernet_ops_rwsem and RTNL,
iterate all netns, and trigger the notifier for all netdev.

Let's hold __rtnl_net_lock() before triggering the notifier.

Note that we will need protection for netdev_chain when RTNL is
removed.  (e.g. blocking_notifier conversion [0] with a lockdep
annotation [1])

Link: https://lore.kernel.org/netdev/20250104063735.36945-2-kuniyu@amazon.com/ [0]
Link: https://lore.kernel.org/netdev/20250105075957.67334-1-kuniyu@amazon.com/

 [1]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250106070751.63146-2-kuniyu@amazon.com


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