helgrind: TestLibRBD.TestIOPP potential deadlock closing an image with read-ahead enabled
If the readahead is the last in-flight IO operation while the image is being closed, it's possible to cause a deadlock attempting to shut down the cache. Since cache read completions hold the cache_lock upon callback, it is required delay the processing of the read completion (i.e. via create_async_context_callback) to prevent such issues. This is handled on the normal IO read path but it is missing on the readahead path.