For our fake dma objects we can leak the underlying gem object if we
fail to pin our "backing storage".

[   39.952618] =============================================================================
[   39.952625] BUG mock_object (Tainted: G     U         ): Objects remaining in mock_object on __kmem_cache_shutdown()
[   39.952629] -----------------------------------------------------------------------------

[   39.952633] Disabling lock debugging due to kernel taint
[   39.952635] INFO: Slab 0xffffea00086c6a00 objects=21 used=1 fp=0xffff88021b1abc00 flags=0x5fff8000008100
[   39.952640] CPU: 1 PID: 1258 Comm: drv_selftest Tainted: G    BU          4.10.0+ #46
[   39.952641] Hardware name: Apple Inc. MacBookPro11,1/Mac-189A3D4F975D5FFC, BIOS MBP111.88Z.0138.B17.1602221718 02/22/2016
[   39.952642] Call Trace:
[   39.952648]  dump_stack+0x4d/0x6f
[   39.952651]  slab_err+0x9d/0xb0
[   39.952654]  ? ksm_migrate_page+0xe0/0xe0
[   39.952657]  ? on_each_cpu_cond+0x9a/0xc0
[   39.952658]  ? __kmalloc+0x1af/0x1c0
[   39.952660]  ? __kmem_cache_shutdown+0x173/0x3e0
[   39.952661]  __kmem_cache_shutdown+0x196/0x3e0
[   39.952664]  kmem_cache_destroy+0xa0/0x150
[   39.952708]  mock_device_release+0x113/0x140 [i915]
[   39.952726]  drm_dev_release+0x20/0x40 [drm]
[   39.952735]  drm_dev_unref+0x23/0x30 [drm]
[   39.952768]  i915_gem_gtt_mock_selftests+0x55/0x70 [i915]
[   39.952803]  __run_selftests+0x169/0x1c0 [i915]
[   39.952805]  ? 0xffffffffa0151000
[   39.952840]  i915_mock_selftests+0x30/0x60 [i915]
[   39.952869]  i915_init+0xc/0x78 [i915]
[   39.952870]  ? 0xffffffffa0151000
[   39.952872]  do_one_initcall+0x43/0x170
[   39.952874]  ? __vunmap+0x81/0xd0
[   39.952875]  ? kmem_cache_alloc_trace+0x37/0x170
[   39.952877]  ? do_init_module+0x27/0x1f8
[   39.952879]  do_init_module+0x5f/0x1f8
[   39.952881]  load_module+0x2423/0x29b0
[   39.952882]  ? __symbol_put+0x40/0x40
[   39.952885]  ? kernel_read_file+0x1a3/0x1c0
[   39.952887]  SYSC_finit_module+0xbc/0xf0
[   39.952889]  SyS_finit_module+0xe/0x10
[   39.952892]  entry_SYSCALL_64_fastpath+0x13/0x94

v2: use onion teardown and favour i915_gem_object_put

Fixes: 8d28ba45 ("drm/i915: Exercise filling the top/bottom portions of the ppgtt")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170306235414.23407-2-matthew.auld@intel.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>