ipmr: Add lockdep expression to ipmr_for_each_table macro

During the initialization process, ipmr_new_table() is called
to create new tables which in turn calls ipmr_get_table() which
traverses net->ipv4.mr_tables without holding the writer lock.
However, this is safe to do so as no tables exist at this time.
Hence add a suitable lockdep expression to silence the following
false-positive warning:

=============================
WARNING: suspicious RCU usage
5.7.0-rc3-next-20200428-syzkaller #0 Not tainted
-----------------------------
net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!

ipmr_get_table+0x130/0x160 net/ipv4/ipmr.c:136
ipmr_new_table net/ipv4/ipmr.c:403 [inline]
ipmr_rules_init net/ipv4/ipmr.c:248 [inline]
ipmr_net_init+0x133/0x430 net/ipv4/ipmr.c:3089

Fixes: f0ad0860 ("ipv4: ipmr: support multiple tables")
Reported-by: syzbot+1519f497f2f9f08183c6@syzkaller.appspotmail.com
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipmr.c

@@ -111,7 +111,8 @@ static void ipmr_expire_process(struct timer_list *t);
 #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
 #define ipmr_for_each_table(mrt, net)					\
 	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
-				lockdep_rtnl_is_held())
+				lockdep_rtnl_is_held() ||		\
+				list_empty(&net->ipv4.mr_tables))
 
 static struct mr_table *ipmr_mr_table_iter(struct net *net,
 					   struct mr_table *mrt)