CVE-2021-47142 drm/amdgpu: Fix a use-after-free

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: Fix a use-after-free

looks like we forget to set ttm->sg to NULL.
Hit panic below

[ 1235.844104] general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b7b4b: 0000 [#1] SMP DEBUG_PAGEALLOC NOPTI
[ 1235.989074] Call Trace:
[ 1235.991751] sg_free_table+0x17/0x20
[ 1235.995667] amdgpu_ttm_backend_unbind.cold+0x4d/0xf7 [amdgpu]
[ 1236.002288] amdgpu_ttm_backend_destroy+0x29/0x130 [amdgpu]
[ 1236.008464] ttm_tt_destroy+0x1e/0x30 [ttm]
[ 1236.013066] ttm_bo_cleanup_memtype_use+0x51/0xa0 [ttm]
[ 1236.018783] ttm_bo_release+0x262/0xa50 [ttm]
[ 1236.023547] ttm_bo_put+0x82/0xd0 [ttm]
[ 1236.027766] amdgpu_bo_unref+0x26/0x50 [amdgpu]
[ 1236.032809] amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu+0x7aa/0xd90 [amdgpu]
[ 1236.040400] kfd_ioctl_alloc_memory_of_gpu+0xe2/0x330 [amdgpu]
[ 1236.046912] kfd_ioctl+0x463/0x690 [amdgpu]