rtnetlink: init handler refcounts to 1

If using CONFIG_REFCOUNT_FULL=y we get following splat:
 refcount_t: increment on 0; use-after-free.
WARNING: CPU: 0 PID: 304 at lib/refcount.c:152 refcount_inc+0x47/0x50
Call Trace:
 rtnetlink_rcv_msg+0x191/0x260
 ...

This warning is harmless (0 is "no callback running", not "memory
was freed").

Use '1' as the new 'no handler is running' base instead of 0 to avoid
this.

Fixes: 019a3169 ("rtnetlink: add reference counting to prevent module unload while dump is in progress")
Reported-by: Sabrina Dubroca <sdubroca@redhat.com>
Reported-by: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -262,7 +262,7 @@ void rtnl_unregister_all(int protocol)
 
 	synchronize_net();
 
-	while (refcount_read(&rtnl_msg_handlers_ref[protocol]) > 0)
+	while (refcount_read(&rtnl_msg_handlers_ref[protocol]) > 1)
 		schedule();
 	kfree(handlers);
 }
@@ -4324,6 +4324,11 @@ static struct pernet_operations rtnetlink_net_ops = {
 
 void __init rtnetlink_init(void)
 {
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(rtnl_msg_handlers_ref); i++)
+		refcount_set(&rtnl_msg_handlers_ref[i], 1);
+
 	if (register_pernet_subsys(&rtnetlink_net_ops))
 		panic("rtnetlink_init: cannot initialize rtnetlink\n");