pppoe: fix memory corruption in padt work structure

pppoe: fix memory corruption in padt work structure

pppoe_connect() mustn't touch the padt_work field of pppoe sockets
because that work could be already pending.

[   21.473147] BUG: unable to handle kernel NULL pointer dereference at 00000004
[   21.474523] IP: [<c1043177>] process_one_work+0x29/0x31c
[   21.475164] *pde = 00000000
[   21.475513] Oops: 0000 [#1] SMP
[   21.475910] Modules linked in: pppoe pppox ppp_generic slhc crc32c_intel aesni_intel virtio_net xts aes_i586 lrw gf128mul ablk_helper cryptd evdev acpi_cpufreq processor serio_raw button ext4 crc16 mbcache jbd2 virtio_blk virtio_pci virtio_ring virtio
[   21.476168] CPU: 2 PID: 164 Comm: kworker/2:2 Not tainted 4.4.0-rc1 #1
[   21.476168] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
[   21.476168] task: f5f83c00 ti: f5e28000 task.ti: f5e28000
[   21.476168] EIP: 0060:[<c1043177>] EFLAGS: 00010046 CPU: 2
[   21.476168] EIP is at process_one_work+0x29/0x31c
[   21.484082] EAX: 00000000 EBX: f678b2a0 ECX: 00000004 EDX: 00000000
[   21.484082] ESI: f6c69940 EDI: f5e29ef0 EBP: f5e29f0c ESP: f5e29edc
[   21.484082]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
[   21.484082] CR0: 80050033 CR2: 000000a4 CR3: 317ad000 CR4: 00040690
[   21.484082] Stack:
[   21.484082]  00000000 f6c69950 00000000 f6c69940 c0042338 f5e29f0c c1327945 00000000
[   21.484082]  00000008 f678b2a0 f6c69940 f678b2b8 f5e29f30 c1043984 f5f83c00 f6c69970
[   21.484082]  f678b2a0 c10437d3 f6775e80 f678b2a0 c10437d3 f5e29fac c1047059 f5e29f74
[   21.484082] Call Trace:
[   21.484082]  [<c1327945>] ? _raw_spin_lock_irq+0x28/0x30
[   21.484082]  [<c1043984>] worker_thread+0x1b1/0x244
[   21.484082]  [<c10437d3>] ? rescuer_thread+0x229/0x229
[   21.484082]  [<c10437d3>] ? rescuer_thread+0x229/0x229
[   21.484082]  [<c1047059>] kthread+0x8f/0x94
[   21.484082]  [<c1327a32>] ? _raw_spin_unlock_irq+0x22/0x26
[   21.484082]  [<c1327ee9>] ret_from_kernel_thread+0x21/0x38
[   21.484082]  [<c1046fca>] ? kthread_parkme+0x19/0x19
[   21.496082] Code: 5d c3 55 89 e5 57 56 53 89 c3 83 ec 24 89 d0 89 55 e0 8d 7d e4 e8 6c d8 ff ff b9 04 00 00 00 89 45 d8 8b 43 24 89 45 dc 8b 45 d8 <8b> 40 04 8b 80 e0 00 00 00 c1 e8 05 24 01 88 45 d7 8b 45 e0 8d
[   21.496082] EIP: [<c1043177>] process_one_work+0x29/0x31c SS:ESP 0068:f5e29edc
[   21.496082] CR2: 0000000000000004
[   21.496082] ---[ end trace e362cc9cf10dae89 ]---

Reported-by: Andrew <nitr0@seti.kr.ua>
Fixes: 287f3a943fef ("pppoe: Use workqueue to die properly when a PADT is received")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>