netfilter: nft_set_rbtree: fix panic when destroying set by GC

netfilter: nft_set_rbtree: fix panic when destroying set by GC

This patch fixes below.
1. check null pointer of rb_next.
 rb_next can return null. so null check routine should be added.
2. add rcu_barrier in destroy routine.
 GC uses call_rcu to remove elements. but all elements should be
 removed before destroying set and chains. so that rcu_barrier is added.

test script:
   %cat test.nft
   table inet aa {
	   map map1 {
		   type ipv4_addr : verdict; flags interval, timeout;
		   elements = {
			   0-1 : jump a0,
			   3-4 : jump a0,
			   6-7 : jump a0,
			   9-10 : jump a0,
			   12-13 : jump a0,
			   15-16 : jump a0,
			   18-19 : jump a0,
			   21-22 : jump a0,
			   24-25 : jump a0,
			   27-28 : jump a0,
		   }
		   timeout 1s;
	   }
	   chain a0 {
	   }
   }
   flush ruleset
   table inet aa {
	   map map1 {
		   type ipv4_addr : verdict; flags interval, timeout;
		   elements = {
			   0-1 : jump a0,
			   3-4 : jump a0,
			   6-7 : jump a0,
			   9-10 : jump a0,
			   12-13 : jump a0,
			   15-16 : jump a0,
			   18-19 : jump a0,
			   21-22 : jump a0,
			   24-25 : jump a0,
			   27-28 : jump a0,
		   }
		   timeout 1s;
	   }
	   chain a0 {
	   }
   }
   flush ruleset

splat looks like:
[ 2402.419838] kasan: GPF could be caused by NULL-ptr deref or user memory access
[ 2402.428433] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[ 2402.429343] CPU: 1 PID: 1350 Comm: kworker/1:1 Not tainted 4.18.0-rc2+ #1
[ 2402.429343] Hardware name: To be filled by O.E.M. To be filled by O.E.M./Aptio CRB, BIOS 5.6.5 03/23/2017
[ 2402.429343] Workqueue: events_power_efficient nft_rbtree_gc [nft_set_rbtree]
[ 2402.429343] RIP: 0010:rb_next+0x1e/0x130
[ 2402.429343] Code: e9 de f2 ff ff 0f 1f 80 00 00 00 00 41 55 48 89 fa 41 54 55 53 48 c1 ea 03 48 b8 00 00 00 0
[ 2402.429343] RSP: 0018:ffff880105f77678 EFLAGS: 00010296
[ 2402.429343] RAX: dffffc0000000000 RBX: ffff8801143e3428 RCX: 1ffff1002287c69c
[ 2402.429343] RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000000
[ 2402.429343] RBP: 0000000000000000 R08: ffffed0016aabc24 R09: ffffed0016aabc24
[ 2402.429343] R10: 0000000000000001 R11: ffffed0016aabc23 R12: 0000000000000000
[ 2402.429343] R13: ffff8800b6933388 R14: dffffc0000000000 R15: ffff8801143e3440
[ 2402.534486] kasan: CONFIG_KASAN_INLINE enabled
[ 2402.534212] FS:  0000000000000000(0000) GS:ffff88011b600000(0000) knlGS:0000000000000000
[ 2402.534212] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2402.534212] CR2: 0000000000863008 CR3: 00000000a3c16000 CR4: 00000000001006e0
[ 2402.534212] Call Trace:
[ 2402.534212]  nft_rbtree_gc+0x2b5/0x5f0 [nft_set_rbtree]
[ 2402.534212]  process_one_work+0xc1b/0x1ee0
[ 2402.540329] kasan: GPF could be caused by NULL-ptr deref or user memory access
[ 2402.534212]  ? _raw_spin_unlock_irq+0x29/0x40
[ 2402.534212]  ? pwq_dec_nr_in_flight+0x3e0/0x3e0
[ 2402.534212]  ? set_load_weight+0x270/0x270
[ 2402.534212]  ? __schedule+0x6ea/0x1fb0
[ 2402.534212]  ? __sched_text_start+0x8/0x8
[ 2402.534212]  ? save_trace+0x320/0x320
[ 2402.534212]  ? sched_clock_local+0xe2/0x150
[ 2402.534212]  ? find_held_lock+0x39/0x1c0
[ 2402.534212]  ? worker_thread+0x35f/0x1150
[ 2402.534212]  ? lock_contended+0xe90/0xe90
[ 2402.534212]  ? __lock_acquire+0x4520/0x4520
[ 2402.534212]  ? do_raw_spin_unlock+0xb1/0x350
[ 2402.534212]  ? do_raw_spin_trylock+0x111/0x1b0
[ 2402.534212]  ? do_raw_spin_lock+0x1f0/0x1f0
[ 2402.534212]  worker_thread+0x169/0x1150

Fixes: 8d8540c4f5e0("netfilter: nft_set_rbtree: add timeout support")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>