net: revert RTNL changes in unregister_netdevice_many_notify()

This patch reverts following changes:

83419b61 net: reduce RTNL hold duration in unregister_netdevice_many_notify() (part 2)
ae646f1a net: reduce RTNL hold duration in unregister_netdevice_many_notify() (part 1)
cfa579f6 net: no longer hold RTNL while calling flush_all_backlogs()

This caused issues in layers holding a private mutex:

cleanup_net()
  rtnl_lock();
	mutex_lock(subsystem_mutex);

	unregister_netdevice();

	   rtnl_unlock();		// LOCKDEP violation
	   rtnl_lock();

I will revisit this in next cycle, opt-in for the new behavior
from safe contexts only.

Fixes: cfa579f6 ("net: no longer hold RTNL while calling flush_all_backlogs()")
Fixes: ae646f1a ("net: reduce RTNL hold duration in unregister_netdevice_many_notify() (part 1)")
Fixes: 83419b61 ("net: reduce RTNL hold duration in unregister_netdevice_many_notify() (part 2)")
Reported-by:  <syzbot+5b9196ecf74447172a9a@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/6789d55f.050a0220.20d369.004e.GAE@google.com/


Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250129142726.747726-1-edumazet@google.com


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