io-wq: clear node->next on list deletion

If someone removes a node from a list, and then later adds it back to
a list, we can have invalid data in ->next. This can cause all sorts
of issues. One such use case is the IORING_OP_POLL_ADD command, which
will do just that if we race and get woken twice without any pending
events. This is a pretty rare case, but can happen under extreme loads.
Dan reports that he saw the following crash:

BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD d283ce067 P4D d283ce067 PUD e5ca04067 PMD 0
Oops: 0002 [#1] SMP
CPU: 17 PID: 10726 Comm: tao:fast-fiber Kdump: loaded Not tainted 5.2.9-02851-gac7bc042d2d1 #116
Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A17 05/03/2019
RIP: 0010:io_wqe_enqueue+0x3e/0xd0
Code: 34 24 74 55 8b 47 58 48 8d 6f 50 85 c0 74 50 48 89 df e8 35 7c 75 00 48 83 7b 08 00 48 8b 14 24 0f 84 84 00 00 00 48 8b 4b 10 <48> 89 11 48 89 53 10 83 63 20 fe 48 89 c6 48 89 df e8 0c 7a 75 00
RSP: 0000:ffffc90006858a08 EFLAGS: 00010082
RAX: 0000000000000002 RBX: ffff889037492fc0 RCX: 0000000000000000
RDX: ffff888e40cc11a8 RSI: ffff888e40cc11a8 RDI: ffff889037492fc0
RBP: ffff889037493010 R08: 00000000000000c3 R09: ffffc90006858ab8
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888e40cc11a8
R13: 0000000000000000 R14: 00000000000000c3 R15: ffff888e40cc1100
FS:  00007fcddc9db700(0000) GS:ffff88903fa40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000e479f5003 CR4: 00000000007606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 <IRQ>
 io_poll_wake+0x12f/0x2a0
 __wake_up_common+0x86/0x120
 __wake_up_common_lock+0x7a/0xc0
 sock_def_readable+0x3c/0x70
 tcp_rcv_established+0x557/0x630
 tcp_v6_do_rcv+0x118/0x3c0
 tcp_v6_rcv+0x97e/0x9d0
 ip6_protocol_deliver_rcu+0xe3/0x440
 ip6_input+0x3d/0xc0
 ? ip6_protocol_deliver_rcu+0x440/0x440
 ipv6_rcv+0x56/0xd0
 ? ip6_rcv_finish_core.isra.18+0x80/0x80
 __netif_receive_skb_one_core+0x50/0x70
 netif_receive_skb_internal+0x2f/0xa0
 napi_gro_receive+0x125/0x150
 mlx5e_handle_rx_cqe+0x1d9/0x5a0
 ? mlx5e_poll_tx_cq+0x305/0x560
 mlx5e_poll_rx_cq+0x49f/0x9c5
 mlx5e_napi_poll+0xee/0x640
 ? smp_reschedule_interrupt+0x16/0xd0
 ? reschedule_interrupt+0xf/0x20
 net_rx_action+0x286/0x3d0
 __do_softirq+0xca/0x297
 irq_exit+0x96/0xa0
 do_IRQ+0x54/0xe0
 common_interrupt+0xf/0xf
 </IRQ>
RIP: 0033:0x7fdc627a2e3a
Code: 31 c0 85 d2 0f 88 f6 00 00 00 55 48 89 e5 41 57 41 56 4c 63 f2 41 55 41 54 53 48 83 ec 18 48 85 ff 0f 84 c7 00 00 00 48 8b 07 <41> 89 d4 49 89 f5 48 89 fb 48 85 c0 0f 84 64 01 00 00 48 83 78 10

when running a networked workload with about 5000 sockets being polled
for. Fix this by clearing node->next when the node is being removed from
the list.

Fixes: 6206f0e1 ("io-wq: shrink io_wq_work a bit")
Reported-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>