rtnetlink: Remove __rtnl_link_register()

rtnetlink: Remove __rtnl_link_register()

link_ops is protected by link_ops_mutex and no longer needs RTNL,
so we have no reason to have __rtnl_link_register() separately.

Let's remove it and call rtnl_link_register() from ifb.ko and
dummy.ko.

Note that both modules' init() work on init_net only, so we need
not export pernet_ops_rwsem and can use rtnl_net_lock() there.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241108004823.29419-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>