drm/ttm: check null pointer before accessing when swapping

Add a check to avoid null pointer dereference as below:

[   90.002283] general protection fault, probably for non-canonical
address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN NOPTI
[   90.002292] KASAN: null-ptr-deref in range
[0x0000000000000000-0x0000000000000007]
[   90.002346]  ? exc_general_protection+0x159/0x240
[   90.002352]  ? asm_exc_general_protection+0x26/0x30
[   90.002357]  ? ttm_bo_evict_swapout_allowable+0x322/0x5e0 [ttm]
[   90.002365]  ? ttm_bo_evict_swapout_allowable+0x42e/0x5e0 [ttm]
[   90.002373]  ttm_bo_swapout+0x134/0x7f0 [ttm]
[   90.002383]  ? __pfx_ttm_bo_swapout+0x10/0x10 [ttm]
[   90.002391]  ? lock_acquire+0x44d/0x4f0
[   90.002398]  ? ttm_device_swapout+0xa5/0x260 [ttm]
[   90.002412]  ? lock_acquired+0x355/0xa00
[   90.002416]  ? do_raw_spin_trylock+0xb6/0x190
[   90.002421]  ? __pfx_lock_acquired+0x10/0x10
[   90.002426]  ? ttm_global_swapout+0x25/0x210 [ttm]
[   90.002442]  ttm_device_swapout+0x198/0x260 [ttm]
[   90.002456]  ? __pfx_ttm_device_swapout+0x10/0x10 [ttm]
[   90.002472]  ttm_global_swapout+0x75/0x210 [ttm]
[   90.002486]  ttm_tt_populate+0x187/0x3f0 [ttm]
[   90.002501]  ttm_bo_handle_move_mem+0x437/0x590 [ttm]
[   90.002517]  ttm_bo_validate+0x275/0x430 [ttm]
[   90.002530]  ? __pfx_ttm_bo_validate+0x10/0x10 [ttm]
[   90.002544]  ? kasan_save_stack+0x33/0x60
[   90.002550]  ? kasan_set_track+0x25/0x30
[   90.002554]  ? __kasan_kmalloc+0x8f/0xa0
[   90.002558]  ? amdgpu_gtt_mgr_new+0x81/0x420 [amdgpu]
[   90.003023]  ? ttm_resource_alloc+0xf6/0x220 [ttm]
[   90.003038]  amdgpu_bo_pin_restricted+0x2dd/0x8b0 [amdgpu]
[   90.003210]  ? __x64_sys_ioctl+0x131/0x1a0
[   90.003210]  ? do_syscall_64+0x60/0x90

Fixes: a2848d08 ("drm/ttm: never consider pinned BOs for eviction&swap")
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20230724024229.1118444-1-guchun.chen@amd.com


Signed-off-by: Christian König <christian.koenig@amd.com>