static ByteBuff getByteBuff(BucketEntry be) {
    return ((BufferGrabbingDeserializer) be.deserializerReference()).buf;
  }
Override the {@link BucketEntry} so that we can set an arbitrary offset.
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/bucket/TestByteBufferIOEngine.java