ipv6: make addrconf_wq single threaded

Both addrconf_verify_work() and addrconf_dad_work() acquire rtnl,
there is no point trying to have one thread per cpu.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240201173031.3654257-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -7349,7 +7349,8 @@ int __init addrconf_init(void)
 	if (err < 0)
 		goto out_addrlabel;
 
-	addrconf_wq = create_workqueue("ipv6_addrconf");
+	/* All works using addrconf_wq need to lock rtnl. */
+	addrconf_wq = create_singlethread_workqueue("ipv6_addrconf");
 	if (!addrconf_wq) {
 		err = -ENOMEM;
 		goto out_nowq;