drm/virtio: fix dma_fence refcount leak on error in virtio_gpu_dma_fence_wait()

In the Linux kernel, the following vulnerability has been resolved: drm/virtio: fix dma_fence refcount leak on error in virtio_gpu_dma_fence_wait() dma_fence_unwrap_for_each() internally calls dma_fence_unwrap_first() which does cursor->chain = dma_fence_get(head), taking an extra reference. On normal loop completion, dma_fence_unwrap_next() releases this via dma_fence_chain_walk() -> dma_fence_put(). When virtio_gpu_do_fence_wait() fails and the function returns early from inside the loop, the cursor->chain reference is never released. This is the only caller in the entire kernel that does an early return inside dma_fence_unwrap_for_each. Add dma_fence_put(itr.chain) before the early return.