drm/amd/amdgpu/amdgpu_cs: fix refcount leak of a dma_fence obj

In the Linux kernel, the following vulnerability has been resolved: drm/amd/amdgpu/amdgpu_cs: fix refcount leak of a dma_fence obj This issue takes place in an error path in amdgpu_cs_fence_to_handle_ioctl(). When `info->in.what` falls into default case, the function simply returns -EINVAL, forgetting to decrement the reference count of a dma_fence obj, which is bumped earlier by amdgpu_cs_get_fence(). This may result in reference count leaks. Fix it by decreasing the refcount of specific object before returning the error code.