staging: usbip: stub_dev: Fixed oops during removal of usbip_host

staging: usbip: stub_dev: Fixed oops during removal of usbip_host

stub_shutdown_connection() should set kernel thread pointers to NULL after killing them.
so that at the time of usbip_host removal stub_shutdown_connection() doesn't try to kill kernel threads
which are already killed.

[ 1504.312158] BUG: unable to handle kernel NULL pointer dereference at           (null)
[ 1504.315833] IP: [<ffffffff8108186f>] exit_creds+0x1f/0x70
[ 1504.317688] PGD 41f1c067 PUD 41d0f067 PMD 0
[ 1504.319611] Oops: 0000 [#2] SMP
[ 1504.321480] 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
[ 1504.329666] CPU 1
[ 1504.329687] Pid: 2434, comm: usbip_eh Tainted: G      D    O 3.6.0-rc31+ #2 LENOVO 20042                           /Base Board Product Name
[ 1504.333502] RIP: 0010:[<ffffffff8108186f>]  [<ffffffff8108186f>] exit_creds+0x1f/0x70
[ 1504.335371] RSP: 0018:ffff880041c7fdd0  EFLAGS: 00010282
[ 1504.337226] RAX: 0000000000000000 RBX: ffff880041c2db40 RCX: ffff880041e4ae50
[ 1504.339101] RDX: 0000000000000044 RSI: 0000000000000286 RDI: 0000000000000000
[ 1504.341027] RBP: ffff880041c7fde0 R08: ffff880041c7e000 R09: 0000000000000000
[ 1504.342934] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[ 1504.344840] R13: 0000000000000000 R14: ffff88004d448000 R15: ffff880041c7fea0
[ 1504.346743] FS:  0000000000000000(0000) GS:ffff880077440000(0000) knlGS:0000000000000000
[ 1504.348671] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 1504.350612] CR2: 0000000000000000 CR3: 0000000041d0d000 CR4: 00000000000007e0
[ 1504.352723] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1504.354734] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1504.356737] Process usbip_eh (pid: 2434, threadinfo ffff880041c7e000, task ffff88004d448000)
[ 1504.358711] Stack:
[ 1504.360635]  ffff880041c7fde0 ffff880041c2db40 ffff880041c7fe00 ffffffff81052aaa
[ 1504.362589]  ffff880041c2db40 0000000000000000 ffff880041c7fe30 ffffffff8107a148
[ 1504.364539]  ffff880041e4ae10 ffff880041e4ae00 ffff88004d448000 ffff88004d448000
[ 1504.366470] Call Trace:
[ 1504.368368]  [<ffffffff81052aaa>] __put_task_struct+0x4a/0x140
[ 1504.370307]  [<ffffffff8107a148>] kthread_stop+0x108/0x110
[ 1504.370312]  [<ffffffffa040da4e>] stub_shutdown_connection+0x3e/0x1b0 [usbip_host]
[ 1504.370315]  [<ffffffffa03fd920>] event_handler_loop+0x70/0x140 [usbip_core]
[ 1504.370318]  [<ffffffff8107ad30>] ? add_wait_queue+0x60/0x60
[ 1504.370320]  [<ffffffffa03fd8b0>] ? usbip_stop_eh+0x30/0x30 [usbip_core]
[ 1504.370322]  [<ffffffff8107a453>] kthread+0x93/0xa0
[ 1504.370327]  [<ffffffff81670e84>] kernel_thread_helper+0x4/0x10
[ 1504.370330]  [<ffffffff8107a3c0>] ? flush_kthread_worker+0xb0/0xb0
[ 1504.370332]  [<ffffffff81670e80>] ? gs_change+0x13/0x13
[ 1504.370351] 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
[ 1504.370353] RIP  [<ffffffff8108186f>] exit_creds+0x1f/0x70
[ 1504.370353]  RSP <ffff880041c7fdd0>
[ 1504.370354] CR2: 0000000000000000
[ 1504.401376] ---[ end trace 1971ce612a16727a ]---

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>