Fix a warning.

v2: Avoid unmapping attachment repeatedly when ERESTARTSYS.

v3: Lock the BO before accessing ttm->sg to avoid race conditions.(Felix)

[   41.708711] WARNING: CPU: 0 PID: 1463 at drivers/gpu/drm/ttm/ttm_bo.c:846 ttm_bo_validate+0x146/0x1b0 [ttm]
[   41.708989] Call Trace:
[   41.708992]  <TASK>
[   41.708996]  ? show_regs+0x6c/0x80
[   41.709000]  ? ttm_bo_validate+0x146/0x1b0 [ttm]
[   41.709008]  ? __warn+0x93/0x190
[   41.709014]  ? ttm_bo_validate+0x146/0x1b0 [ttm]
[   41.709024]  ? report_bug+0x1f9/0x210
[   41.709035]  ? handle_bug+0x46/0x80
[   41.709041]  ? exc_invalid_op+0x1d/0x80
[   41.709048]  ? asm_exc_invalid_op+0x1f/0x30
[   41.709057]  ? amdgpu_amdkfd_gpuvm_dmaunmap_mem+0x2c/0x80 [amdgpu]
[   41.709185]  ? ttm_bo_validate+0x146/0x1b0 [ttm]
[   41.709197]  ? amdgpu_amdkfd_gpuvm_dmaunmap_mem+0x2c/0x80 [amdgpu]
[   41.709337]  ? srso_alias_return_thunk+0x5/0x7f
[   41.709346]  kfd_mem_dmaunmap_attachment+0x9e/0x1e0 [amdgpu]
[   41.709467]  amdgpu_amdkfd_gpuvm_dmaunmap_mem+0x56/0x80 [amdgpu]
[   41.709586]  kfd_ioctl_unmap_memory_from_gpu+0x1b7/0x300 [amdgpu]
[   41.709710]  kfd_ioctl+0x1ec/0x650 [amdgpu]
[   41.709822]  ? __pfx_kfd_ioctl_unmap_memory_from_gpu+0x10/0x10 [amdgpu]
[   41.709945]  ? srso_alias_return_thunk+0x5/0x7f
[   41.709949]  ? tomoyo_file_ioctl+0x20/0x30
[   41.709959]  __x64_sys_ioctl+0x9c/0xd0
[   41.709967]  do_syscall_64+0x3f/0x90
[   41.709973]  entry_SYSCALL_64_after_hwframe+0x6e/0xd8

Fixes: 101b8104 ("drm/amdkfd: Move dma unmapping after TLB flush")
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>