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.