netfilter: nf_tables: silence a RCU-list warning in nft_table_lookup()

It is safe to traverse &net->nft.tables with &net->nft.commit_mutex
held using list_for_each_entry_rcu(). Silence the PROVE_RCU_LIST false
positive,

WARNING: suspicious RCU usage
net/netfilter/nf_tables_api.c:523 RCU-list traversed in non-reader section!!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by iptables/1384:
 #0: ffffffff9745c4a8 (&net->nft.commit_mutex){+.+.}, at: nf_tables_valid_genid+0x25/0x60 [nf_tables]

Call Trace:
 dump_stack+0xa1/0xea
 lockdep_rcu_suspicious+0x103/0x10d
 nft_table_lookup.part.0+0x116/0x120 [nf_tables]
 nf_tables_newtable+0x12c/0x7d0 [nf_tables]
 nfnetlink_rcv_batch+0x559/0x1190 [nfnetlink]
 nfnetlink_rcv+0x1da/0x210 [nfnetlink]
 netlink_unicast+0x306/0x460
 netlink_sendmsg+0x44b/0x770
 ____sys_sendmsg+0x46b/0x4a0
 ___sys_sendmsg+0x138/0x1a0
 __sys_sendmsg+0xb6/0x130
 __x64_sys_sendmsg+0x48/0x50
 do_syscall_64+0x69/0xf4
 entry_SYSCALL_64_after_hwframe+0x49/0xb3
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -520,7 +520,8 @@ static struct nft_table *nft_table_lookup(const struct net *net,
 	if (nla == NULL)
 		return ERR_PTR(-EINVAL);
 
-	list_for_each_entry_rcu(table, &net->nft.tables, list) {
+	list_for_each_entry_rcu(table, &net->nft.tables, list,
+				lockdep_is_held(&net->nft.commit_mutex)) {
 		if (!nla_strcmp(nla, table->name) &&
 		    table->family == family &&
 		    nft_active_genmask(table, genmask))