rtnetlink: Panic when __rtnl_register_many() fails for builtin callers.

We will replace all rtnl_register() and rtnl_register_module() with
rtnl_register_many().

Currently, rtnl_register() returns nothing and prints an error message
when it fails to register a rtnetlink message type and handlers.

The failure happens only when rtnl_register_internal() fails to allocate
rtnl_msg_handlers[protocol][msgtype], but it's unlikely for built-in
callers on boot time.

rtnl_register_many() unwinds the previous successful registrations on
failure and returns an error, but it will be useless for built-in callers,
especially some subsystems that do not have the legacy ioctl() interface
and do not work without rtnetlink.

Instead of booting up without rtnetlink functionality, let's panic on
failure for built-in rtnl_register_many() callers.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241014201828.91221-2-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -464,6 +464,10 @@ int __rtnl_register_many(const struct rtnl_msg_handler *handlers, int n)
 					     handler->msgtype, handler->doit,
 					     handler->dumpit, handler->flags);
 		if (err) {
+			if (!handler->owner)
+				panic("Unable to register rtnetlink message "
+				      "handlers, %pS\n", handlers);
+
 			__rtnl_unregister_many(handlers, i);
 			break;
 		}