rtnetlink: fallback to UNSPEC if current family has no doit callback

We need to use PF_UNSPEC in case the requested family has no doit
callback, otherwise this now fails with EOPNOTSUPP instead of running the
unspec doit callback, as before.

Fixes: 6853dd488119 ("rtnetlink: protect handler table with rcu")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -4221,6 +4221,12 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return err;
 	}
 
+	doit = READ_ONCE(handlers[type].doit);
+	if (!doit) {
+		family = PF_UNSPEC;
+		handlers = rcu_dereference(rtnl_msg_handlers[family]);
+	}
+
 	flags = READ_ONCE(handlers[type].flags);
 	if (flags & RTNL_FLAG_DOIT_UNLOCKED) {
 		refcount_inc(&rtnl_msg_handlers_ref[family]);