GlusterVolume modification to wrap getPos() functionality in underlying ...

This PullRequest Wraps calls to get pos and records the last value, returning that last value in the case that the underlying stream closes.  This fix is essential for distributed mapreduce jobs that use the mapred.* API, because that API expects a working call to FSInputStream.getPos() even after the stream has been read:  And because the RawLocalFileSystem buffers that stream (unlike DFSInputStream, which handles doesnt wrap the stream), the buffer closes and sets the underlying stream to null .  See BZ 100362 for details.