drm/ttm: Fix dma_fence refcnt leak in ttm_bo_vm_fault_reserved

ttm_bo_vm_fault_reserved() invokes dma_fence_get(), which returns a
reference of the specified dma_fence object to "moving" with increased
refcnt.

When ttm_bo_vm_fault_reserved() returns, local variable "moving" becomes
invalid, so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in several exception handling paths
of ttm_bo_vm_fault_reserved(). When those error scenarios occur such as
"err" equals to -EBUSY, the function forgets to decrease the refcnt
increased by dma_fence_get(), causing a refcnt leak.

Fix this issue by calling dma_fence_put() when no_wait_gpu flag is
equals to true.
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/370219/Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -300,8 +300,10 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
 			break;
 		case -EBUSY:
 		case -ERESTARTSYS:
+			dma_fence_put(moving);
 			return VM_FAULT_NOPAGE;
 		default:
+			dma_fence_put(moving);
 			return VM_FAULT_SIGBUS;
 		}