Highly concurrent Piglit runs can trigger a race condition where a pending
SDMA job on a buffer object is never executed because the corresponding
process is killed (perhaps due to a crash). Since the job's fences were
never signaled, the buffer object was effectively leaked. Worse, the
buffer was stuck wherever it happened to be at the time, possibly in VRAM.

The symptom was user space processes stuck in interruptible waits with
kernel stacks like:

    [<ffffffffbc5e6722>] dma_fence_default_wait+0x112/0x250
    [<ffffffffbc5e6399>] dma_fence_wait_timeout+0x39/0xf0
    [<ffffffffbc5e82d2>] reservation_object_wait_timeout_rcu+0x1c2/0x300
    [<ffffffffc03ce56f>] ttm_bo_cleanup_refs_and_unlock+0xff/0x1a0 [ttm]
    [<ffffffffc03cf1ea>] ttm_mem_evict_first+0xba/0x1a0 [ttm]
    [<ffffffffc03cf611>] ttm_bo_mem_space+0x341/0x4c0 [ttm]
    [<ffffffffc03cfc54>] ttm_bo_validate+0xd4/0x150 [ttm]
    [<ffffffffc03cffbd>] ttm_bo_init_reserved+0x2ed/0x420 [ttm]
    [<ffffffffc042f523>] amdgpu_bo_create_restricted+0x1f3/0x470 [amdgpu]
    [<ffffffffc042f9fa>] amdgpu_bo_create+0xda/0x220 [amdgpu]
    [<ffffffffc04349ea>] amdgpu_gem_object_create+0xaa/0x140 [amdgpu]
    [<ffffffffc0434f97>] amdgpu_gem_create_ioctl+0x97/0x120 [amdgpu]
    [<ffffffffc037ddba>] drm_ioctl+0x1fa/0x480 [drm]
    [<ffffffffc041904f>] amdgpu_drm_ioctl+0x4f/0x90 [amdgpu]
    [<ffffffffbc23db33>] do_vfs_ioctl+0xa3/0x5f0
    [<ffffffffbc23e0f9>] SyS_ioctl+0x79/0x90
    [<ffffffffbc864ffb>] entry_SYSCALL_64_fastpath+0x1e/0xad
    [<ffffffffffffffff>] 0xffffffffffffffff

Note: The correctness of this change depends on the earlier commit
"drm/amd/sched: move adding finish callback to amd_sched_job_begin"

v2: set an error on the finished fence
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>