drm/i915/selftests: Hold task_struct ref for smoking kthread

As the kthread may terminate itself, the parent must hold a task_struct
reference for it to call kthread_stop().

<4> [498.827675] stack segment: 0000 [#1] PREEMPT SMP PTI
<4> [498.827683] CPU: 0 PID: 3872 Comm: drv_selftest Tainted: G     U            4.19.0-rc6-CI-CI_DRM_4915+ #1
<4> [498.827686] Hardware name: Intel Corporation NUC7CJYH/NUC7JYB, BIOS JYGLKCPX.86A.0027.2018.0125.1347 01/25/2018
<4> [498.827695] RIP: 0010:kthread_stop+0x36/0x210
<4> [498.827698] Code: 05 df 3d f6 7e 89 c0 48 0f a3 05 95 f8 29 01 0f 82 56 01 00 00 f0 ff 43 20 f6 43 26 20 0f 84 7f 01 00 00 48 8b ab b0 05 00 00 <f0> 80 4d 00 02 48 89 df e8 5d ff ff ff 48 89 df e8 15 c7 00 00 48
<4> [498.827701] RSP: 0018:ffffc900003937d0 EFLAGS: 00010202
<4> [498.827704] RAX: 0000000000000001 RBX: ffff8802165ece40 RCX: 0000000000000001
<4> [498.827707] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffffffff82247460
<4> [498.827709] RBP: 6b6b6b6b6b6b6b6b R08: 00000000581395cb R09: 0000000000000001
<4> [498.827711] R10: 0000000000000000 R11: 0000000000000000 R12: ffffc90000393868
<4> [498.827713] R13: ffffc900003937f0 R14: ffff88026c068040 R15: 0000000000001057
<4> [498.827716] FS:  00007fc0c464b980(0000) GS:ffff880277e00000(0000) knlGS:0000000000000000
<4> [498.827718] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [498.827720] CR2: 000056178c2feca0 CR3: 000000026983c000 CR4: 0000000000340ef0
<4> [498.827723] Call Trace:
<4> [498.827824]  smoke_crescendo+0x14c/0x1d0 [i915]
<4> [498.827837]  ? _raw_spin_unlock_irqrestore+0x4c/0x60
<4> [498.827898]  ? __i915_gem_context_pin_hw_id+0x69/0x5f0 [i915]
<4> [498.827902]  ? ida_alloc_range+0x1f2/0x3d0
<4> [498.827907]  ? __mutex_unlock_slowpath+0x46/0x2b0
<4> [498.827914]  ? rcu_lockdep_current_cpu_online+0x8f/0xd0
<4> [498.827979]  live_preempt_smoke+0x2c2/0x470 [i915]
<4> [498.828047]  __i915_subtests+0x5e/0xf0 [i915]
<4> [498.828113]  __run_selftests+0x10b/0x190 [i915]
<4> [498.828175]  i915_live_selftests+0x2c/0x60 [i915]
<4> [498.828232]  i915_pci_probe+0x50/0xa0 [i915]
<4> [498.828238]  pci_device_probe+0xa1/0x130
<4> [498.828244]  really_probe+0x25d/0x3c0
<4> [498.828249]  driver_probe_device+0x10a/0x120
<4> [498.828253]  __driver_attach+0xdb/0x100
<4> [498.828256]  ? driver_probe_device+0x120/0x120
<4> [498.828259]  bus_for_each_dev+0x74/0xc0
<4> [498.828264]  bus_add_driver+0x15f/0x250
<4> [498.828268]  ? 0xffffffffa00c3000
<4> [498.828271]  driver_register+0x56/0xe0
<4> [498.828274]  ? 0xffffffffa00c3000
<4> [498.828278]  do_one_initcall+0x58/0x2e0
<4> [498.828281]  ? rcu_lockdep_current_cpu_online+0x8f/0xd0
<4> [498.828285]  ? do_init_module+0x1d/0x1ea
<4> [498.828289]  ? rcu_read_lock_sched_held+0x6f/0x80
<4> [498.828293]  ? kmem_cache_alloc_trace+0x264/0x290
<4> [498.828297]  do_init_module+0x56/0x1ea
<4> [498.828302]  load_module+0x26f5/0x29d0
<4> [498.828309]  ? vfs_read+0x122/0x140
<4> [498.828318]  ? __se_sys_finit_module+0xd3/0xf0
<4> [498.828321]  __se_sys_finit_module+0xd3/0xf0
<4> [498.828329]  do_syscall_64+0x55/0x190
<4> [498.828332]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
<4> [498.828335] RIP: 0033:0x7fc0c3f16839

Fixes: 992d2098 ("drm/i915/selftests: Split preemption smoke test into threads")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002132927.7669-1-chris@chris-wilson.co.uk