In response to "usbip detach -p [port_number]" user command,vhci_shutdown_connection() gets
executed which kills tcp_tx,tcp_rx kernel threads but doesn't set thread pointers to NULL.
so, at the time of vhci_hcd removal vhci_shutdown_connection() again tries to kill kernel
threads which are already killed.

[  312.220259] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  312.220313] IP: [<ffffffff8108186f>] exit_creds+0x1f/0x70
[  312.220349] PGD 5b7be067 PUD 5b7dc067 PMD 0
[  312.220388] Oops: 0000 [#1] SMP
[  312.220415] Modules linked in: vhci_hcd(O-) usbip_host(O) usbip_core(O) uas usb_storage joydev parport_pc bnep rfcomm ppdev binfmt_misc
snd_hda_codec_hdmi snd_hda_codec_conexant snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_seq_midi arc4 snd_rawmidi snd_seq_midi_event snd_seq
ath9k mac80211 ath9k_common ath9k_hw snd_timer snd_seq_device snd ath i915 drm_kms_helper drm psmouse cfg80211 coretemp soundcore lpc_ich i2c_algo_bit
snd_page_alloc video intel_ips wmi btusb mac_hid bluetooth ideapad_laptop lp sparse_keymap serio_raw mei microcode parport r8169
[  312.220862] CPU 0
[  312.220882] Pid: 2095, comm: usbip_eh Tainted: G           O 3.6.0-rc3+ #2 LENOVO 20042                           /Base Board Product Name
[  312.220938] RIP: 0010:[<ffffffff8108186f>]  [<ffffffff8108186f>] exit_creds+0x1f/0x70
[  312.220979] RSP: 0018:ffff88004d6ebdb0  EFLAGS: 00010282
[  312.221008] RAX: 0000000000000000 RBX: ffff880041c4db40 RCX: ffff88005ad52230
[  312.221041] RDX: 0000000000000034 RSI: 0000000000000286 RDI: 0000000000000000
[  312.221074] RBP: ffff88004d6ebdc0 R08: ffff88004d6ea000 R09: 0000000000000000
[  312.221107] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[  312.221144] R13: 0000000000000000 R14: ffff88005ad521d8 R15: ffff88004d6ebea0
[  312.221177] FS:  0000000000000000(0000) GS:ffff880077400000(0000) knlGS:0000000000000000
[  312.221214] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  312.221241] CR2: 0000000000000000 CR3: 000000005b7b9000 CR4: 00000000000007f0
[  312.221277] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  312.221309] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  312.221342] Process usbip_eh (pid: 2095, threadinfo ffff88004d6ea000, task ffff88004d44db40)
[  312.221384] Stack:
[  312.221396]  ffff88004d6ebdc0 ffff880041c4db40 ffff88004d6ebde0 ffffffff81052aaa
[  312.221442]  ffff880041c4db40 0000000000000000 ffff88004d6ebe10 ffffffff8107a148
[  312.221487]  ffff88005ad521f0 ffff88005ad52228 ffff88004d44db40 ffff88005ad521d8
[  312.221536] Call Trace:
[  312.221556]  [<ffffffff81052aaa>] __put_task_struct+0x4a/0x140
[  312.221587]  [<ffffffff8107a148>] kthread_stop+0x108/0x110
[  312.221616]  [<ffffffffa0412569>] vhci_shutdown_connection+0x49/0x2b4 [vhci_hcd]
[  312.221657]  [<ffffffffa0139920>] event_handler_loop+0x70/0x140 [usbip_core]
[  312.221692]  [<ffffffff8107ad30>] ? add_wait_queue+0x60/0x60
[  312.221721]  [<ffffffffa01398b0>] ? usbip_stop_eh+0x30/0x30 [usbip_core]
[  312.221754]  [<ffffffff8107a453>] kthread+0x93/0xa0
[  312.221784]  [<ffffffff81670e84>] kernel_thread_helper+0x4/0x10
[  312.221814]  [<ffffffff8107a3c0>] ? flush_kthread_worker+0xb0/0xb0
[  312.223589]  [<ffffffff81670e80>] ? gs_change+0x13/0x13
[  312.225360] Code: 0b 0f 0b 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 53 48 83 ec 08 66 66 66 66 90 48 8b 87 58 04 00 00 48 89 fb 48 8b bf 50 04 00 00
<8b> 00 48 c7 83 50 04 00 00 00 00 00 00 f0 ff 0f 0f 94 c0 84 c0
[  312.229663] RIP  [<ffffffff8108186f>] exit_creds+0x1f/0x70
[  312.231696]  RSP <ffff88004d6ebdb0>
[  312.233648] CR2: 0000000000000000
[  312.256464] ---[ end trace 1971ce612a167279 ]---
Signed-off-by: navin patidar <navinp@cdac.in>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>