8213927: G1 ignores AlwaysPreTouch when UseTransparentHugePages is enabled

With UseTransparentHugePages we always need to pretouch on small page size as the underlying memory could currently be allocated as either small or large pages.

Reviewed-by: shade, sjohanss

src/hotspot/share/gc/g1/g1PageBasedVirtualSpace.cpp

@@ -232,14 +232,19 @@ private:
   char* volatile _cur_addr;
   char* const _start_addr;
   char* const _end_addr;
-  size_t const _page_size;
+  size_t _page_size;
 public:
   G1PretouchTask(char* start_address, char* end_address, size_t page_size) :
     AbstractGangTask("G1 PreTouch"),
     _cur_addr(start_address),
     _start_addr(start_address),
     _end_addr(end_address),
-    _page_size(page_size) {
+    _page_size(0) {
+#ifdef LINUX
+    _page_size = UseTransparentHugePages ? (size_t)os::vm_page_size(): page_size;
+#else
+    _page_size = page_size;
+#endif
   }
 
   virtual void work(uint worker_id) {