dma-buf needs a fence object

As one of several types of synchronization objects, this object would need to:

    - Be a separate object with a file descriptor that is returned to user space (should be selectable to avoid polling).
    - Have some extension for asynchronous notification for receiving completion event on kernel side (eventfd might be a good starting point)
    - Have a user-space API for passing a sync-obj file descriptor (Android has ready solution, DRI2 for general Linux?)
    - DRI2 type use might need to have sync-obj as property of 'struct dma_buf'.
    - Question: is sync-obj a transient thing? If so, how would that work as a property of 'struct dma_buf' (separation of sync obj and sync point...)