Set ttm->sg to NULL after kfree, to avoid memory corruption backtrace:

[  420.932812] kernel BUG at
/build/linux-do9eLF/linux-4.15.0/mm/slub.c:295!
[  420.934182] invalid opcode: 0000 [#1] SMP NOPTI
[  420.935445] Modules linked in: xt_conntrack ipt_MASQUERADE
[  420.951332] Hardware name: Dell Inc. PowerEdge R7525/0PYVT1, BIOS
1.5.4 07/09/2020
[  420.952887] RIP: 0010:__slab_free+0x180/0x2d0
[  420.954419] RSP: 0018:ffffbe426291fa60 EFLAGS: 00010246
[  420.955963] RAX: ffff9e29263e9c30 RBX: ffff9e29263e9c30 RCX:
000000018100004b
[  420.957512] RDX: ffff9e29263e9c30 RSI: fffff3d33e98fa40 RDI:
ffff9e297e407a80
[  420.959055] RBP: ffffbe426291fb00 R08: 0000000000000001 R09:
ffffffffc0d39ade
[  420.960587] R10: ffffbe426291fb20 R11: ffff9e49ffdd4000 R12:
ffff9e297e407a80
[  420.962105] R13: fffff3d33e98fa40 R14: ffff9e29263e9c30 R15:
ffff9e2954464fd8
[  420.963611] FS:  00007fa2ea097780(0000) GS:ffff9e297e840000(0000)
knlGS:0000000000000000
[  420.965144] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  420.966663] CR2: 00007f16bfffefb8 CR3: 0000001ff0c62000 CR4:
0000000000340ee0
[  420.968193] Call Trace:
[  420.969703]  ? __page_cache_release+0x3c/0x220
[  420.971294]  ? amdgpu_ttm_tt_unpopulate+0x5e/0x80 [amdgpu]
[  420.972789]  kfree+0x168/0x180
[  420.974353]  ? amdgpu_ttm_tt_set_user_pages+0x64/0xc0 [amdgpu]
[  420.975850]  ? kfree+0x168/0x180
[  420.977403]  amdgpu_ttm_tt_unpopulate+0x5e/0x80 [amdgpu]
[  420.978888]  ttm_tt_unpopulate.part.10+0x53/0x60 [amdttm]
[  420.980357]  ttm_tt_destroy.part.11+0x4f/0x60 [amdttm]
[  420.981814]  ttm_tt_destroy+0x13/0x20 [amdttm]
[  420.983273]  ttm_bo_cleanup_memtype_use+0x36/0x80 [amdttm]
[  420.984725]  ttm_bo_release+0x1c9/0x360 [amdttm]
[  420.986167]  amdttm_bo_put+0x24/0x30 [amdttm]
[  420.987663]  amdgpu_bo_unref+0x1e/0x30 [amdgpu]
[  420.989165]  amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu+0x9ca/0xb10
[amdgpu]
[  420.990666]  kfd_ioctl_alloc_memory_of_gpu+0xef/0x2c0 [amdgpu]
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>