[FIX] fix plugin system

plugin/README.md

@@ -8,7 +8,7 @@ To include a certain plugin, say ```plugin_a```, you only need to add the follow
 
 ```makefile
 # Add plugin by include the plugin in config
-include plugin/plugin_a/plugin.mk
+XGB_PLUGINS += plugin/plugin_a/plugin.mk
 ```
 
 Then rebuild libxgboost by typing make, you can get a new library with the plugin enabled.

plugin/example/README.md

@@ -12,7 +12,7 @@ There are three steps you need to to do to add plugin to xgboost
 To add this plugin, add the following line to ```config.mk```(template in make/config.mk).
 ```makefile
 # Add plugin by include the plugin in config
-include plugin/example/plugin.mk
+XGB_PLUGINS += plugin/plugin_a/plugin.mk
 ```
 
 Then you can test this plugin by using ```objective=mylogistic``` parameter.

plugin/lz4/sparse_page_lz4_format.cc

@@ -104,7 +104,8 @@ inline void CompressArray<DType>::InitCompressChunks(size_t chunk_size, size_t m
   chunk_size = std::max(min_chunk_size, chunk_size);
   size_t nstep = data.size() / chunk_size;
   for (size_t i = 0; i < nstep; ++i) {
-    raw_chunks_.push_back(raw_chunks_.back() + chunk_size * i);
+    raw_chunks_.push_back(raw_chunks_.back() + chunk_size);
+    CHECK_LE(raw_chunks_.back(), data.size());
   }
   if (nstep == 0) raw_chunks_.push_back(0);
   raw_chunks_.back() = data.size();