netfilter: nf_tables: make table handle allocation per-netns friendly

mutex is per-netns, move table_netns to the pernet area.

*read-write* to 0xffffffff883a01e8 of 8 bytes by task 6542 on cpu 0:
 nf_tables_newtable+0x6dc/0xc00 net/netfilter/nf_tables_api.c:1221
 nfnetlink_rcv_batch net/netfilter/nfnetlink.c:513 [inline]
 nfnetlink_rcv_skb_batch net/netfilter/nfnetlink.c:634 [inline]
 nfnetlink_rcv+0xa6a/0x13a0 net/netfilter/nfnetlink.c:652
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x652/0x730 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x643/0x740 net/netlink/af_netlink.c:1921

Fixes: f102d66b ("netfilter: nf_tables: use dedicated mutex to guard transactions")
Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -1652,6 +1652,7 @@ struct nftables_pernet {
 	struct list_head	module_list;
 	struct list_head	notify_list;
 	struct mutex		commit_mutex;
+	u64			table_handle;
 	unsigned int		base_seq;
 	u8			validate_state;
 };

net/netfilter/nf_tables_api.c

@@ -32,7 +32,6 @@ static LIST_HEAD(nf_tables_objects);
 static LIST_HEAD(nf_tables_flowtables);
 static LIST_HEAD(nf_tables_destroy_list);
 static DEFINE_SPINLOCK(nf_tables_destroy_list_lock);
-static u64 table_handle;
 
 enum {
 	NFT_VALIDATE_SKIP	= 0,
@@ -1235,7 +1234,7 @@ static int nf_tables_newtable(struct sk_buff *skb, const struct nfnl_info *info,
 	INIT_LIST_HEAD(&table->flowtables);
 	table->family = family;
 	table->flags = flags;
-	table->handle = ++table_handle;
+	table->handle = ++nft_net->table_handle;
 	if (table->flags & NFT_TABLE_F_OWNER)
 		table->nlpid = NETLINK_CB(skb).portid;