drm/amdgpu/userq: Fix fence reference leak on queue teardown v2

drm/amdgpu/userq: Fix fence reference leak on queue teardown v2

The user mode queue keeps a pointer to the most recent fence in
userq->last_fence. This pointer holds an extra dma_fence reference.

When the queue is destroyed, we free the fence driver and its xarray,
but we forgot to drop the last_fence reference.

Because of the missing dma_fence_put(), the last fence object can stay
alive when the driver unloads. This leaves an allocated object in the
amdgpu_userq_fence slab cache and triggers

This is visible during driver unload as:

  BUG amdgpu_userq_fence: Objects remaining on __kmem_cache_shutdown()
  kmem_cache_destroy amdgpu_userq_fence: Slab cache still has objects
  Call Trace:
    kmem_cache_destroy
    amdgpu_userq_fence_slab_fini
    amdgpu_exit
    __do_sys_delete_module

Fix this by putting userq->last_fence and clearing the pointer during
amdgpu_userq_fence_driver_free().

This makes sure the fence reference is released and the slab cache is
empty when the module exits.

v2: Update to only release userq->last_fence with dma_fence_put()
    (Christian)

Fixes: edc762a51c71 ("drm/amdgpu/userq: move some code around")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>