MINOR: compression: tune.comp.maxlevel

This option allows you to set the maximum compression level usable by
the compression algorithm. It affects CPU usage.
diff --git a/include/types/compression.h b/include/types/compression.h
index fa251f1..da356ad 100644
--- a/include/types/compression.h
+++ b/include/types/compression.h
@@ -44,6 +44,7 @@
 	void *zlib_pending_buf;
 	void *zlib_head;
 #endif /* USE_ZLIB */
+	int cur_lvl;
 };
 
 struct comp_algo {