The nft_set_gc_batch_check() checks whether gc buffer is full.
If gc buffer is full, gc buffer is released by
the nft_set_gc_batch_complete() internally.
In case of rbtree, the rb_erase() should be called before calling the
nft_set_gc_batch_complete(). therefore the rb_erase() should
be called before calling the nft_set_gc_batch_check() too.

test commands:
   table ip filter {
	   set set1 {
		   type ipv4_addr; flags interval, timeout;
		   gc-interval 10s;
		   timeout 1s;
		   elements = {
			   1-2,
			   3-4,
			   5-6,
			   ...
			   10000-10001,
		   }
	   }
   }
   %nft -f test.nft

splat looks like:
[  430.273885] kasan: GPF could be caused by NULL-ptr deref or user memory access
[  430.282158] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  430.283116] CPU: 1 PID: 190 Comm: kworker/1:2 Tainted: G    B             4.18.0+ #7
[  430.283116] Workqueue: events_power_efficient nft_rbtree_gc [nf_tables_set]
[  430.313559] RIP: 0010:rb_next+0x81/0x130
[  430.313559] Code: 08 49 bd 00 00 00 00 00 fc ff df 48 bb 00 00 00 00 00 fc ff df 48 85 c0 75 05 eb 58 48 89 d4
[  430.313559] RSP: 0018:ffff88010cdb7680 EFLAGS: 00010207
[  430.313559] RAX: 0000000000b84854 RBX: dffffc0000000000 RCX: ffffffff83f01973
[  430.313559] RDX: 000000000017090c RSI: 0000000000000008 RDI: 0000000000b84864
[  430.313559] RBP: ffff8801060d4588 R08: fffffbfff09bc349 R09: fffffbfff09bc349
[  430.313559] R10: 0000000000000001 R11: fffffbfff09bc348 R12: ffff880100f081a8
[  430.313559] R13: dffffc0000000000 R14: ffff880100ff8688 R15: dffffc0000000000
[  430.313559] FS:  0000000000000000(0000) GS:ffff88011b400000(0000) knlGS:0000000000000000
[  430.313559] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  430.313559] CR2: 0000000001551008 CR3: 000000005dc16000 CR4: 00000000001006e0
[  430.313559] Call Trace:
[  430.313559]  nft_rbtree_gc+0x112/0x5c0 [nf_tables_set]
[  430.313559]  process_one_work+0xc13/0x1ec0
[  430.313559]  ? _raw_spin_unlock_irq+0x29/0x40
[  430.313559]  ? pwq_dec_nr_in_flight+0x3c0/0x3c0
[  430.313559]  ? set_load_weight+0x270/0x270
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x40/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x40/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x40/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __switch_to_asm+0x40/0x70
[  430.313559]  ? __switch_to_asm+0x34/0x70
[  430.313559]  ? __schedule+0x6d3/0x1f50
[  430.313559]  ? find_held_lock+0x39/0x1c0
[  430.313559]  ? __sched_text_start+0x8/0x8
[  430.313559]  ? cyc2ns_read_end+0x10/0x10
[  430.313559]  ? save_trace+0x300/0x300
[  430.313559]  ? sched_clock_local+0xd4/0x140
[  430.313559]  ? find_held_lock+0x39/0x1c0
[  430.313559]  ? worker_thread+0x353/0x1120
[  430.313559]  ? worker_thread+0x353/0x1120
[  430.313559]  ? lock_contended+0xe70/0xe70
[  430.313559]  ? __lock_acquire+0x4500/0x4500
[  430.535635]  ? do_raw_spin_unlock+0xa5/0x330
[  430.535635]  ? do_raw_spin_trylock+0x101/0x1a0
[  430.535635]  ? do_raw_spin_lock+0x1f0/0x1f0
[  430.535635]  ? _raw_spin_lock_irq+0x10/0x70
[  430.535635]  worker_thread+0x15d/0x1120
[ ... ]

Fixes: 8d8540c4 ("netfilter: nft_set_rbtree: add timeout support")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>