Add a method to get file length for Seekable, FSDataInputStream and libhdfs

When open any seekable file, it should be able to get the length of the file via Seekable interface, since the seek method should be able to detect seeking beyond the end of file. Such interface can benefit distributed file systems by saving a network round-trip of FileSystem.getFileStatus(Path).getLen() for any open file.
In libhdfs, such interface should also be exposed to make native program taking advantage of this change.
I have the changes locally for all FSInputStream concrete classes. The change can be considered trivial, since some of the FSInputStream classes already have a method named getFileLength(), or a member field named size/length/end.