ip6mr: Fix RCU list debugging warning

ip6mr_for_each_table() macro uses list_for_each_entry_rcu()
for traversing outside an RCU read side critical section
but under the protection of rtnl_mutex. Hence add the
corresponding lockdep expression to silence the following
false-positive warnings:

[    4.319479] =============================
[    4.319480] WARNING: suspicious RCU usage
[    4.319482] 5.5.4-stable #17 Tainted: G            E
[    4.319483] -----------------------------
[    4.319485] net/ipv6/ip6mr.c:1243 RCU-list traversed in non-reader section!!

[    4.456831] =============================
[    4.456832] WARNING: suspicious RCU usage
[    4.456834] 5.5.4-stable #17 Tainted: G            E
[    4.456835] -----------------------------
[    4.456837] net/ipv6/ip6mr.c:1582 RCU-list traversed in non-reader section!!
Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -97,7 +97,8 @@ static void ipmr_expire_process(struct timer_list *t);
 
 #ifdef CONFIG_IPV6_MROUTE_MULTIPLE_TABLES
 #define ip6mr_for_each_table(mrt, net) \
-	list_for_each_entry_rcu(mrt, &net->ipv6.mr6_tables, list)
+	list_for_each_entry_rcu(mrt, &net->ipv6.mr6_tables, list, \
+				lockdep_rtnl_is_held())
 
 static struct mr_table *ip6mr_mr_table_iter(struct net *net,
 					    struct mr_table *mrt)