Enable guard pages in CHECK_CLIENT

Change the behaviour to use PROT_NONE for reservations in CHECK_CLIENT
mode.  This means that we only provide access once data is actually
being used.

src/mem/metaslab.h

@@ -30,7 +30,7 @@ namespace snmalloc
      *
      *  Spare 32bits are used for the fields in MetaslabEnd.
      */
-#ifdef CHECK_CLIENT
+#ifdef SNMALLOC_CHECK_CLIENT
     FreeListBuilder<true> free_queue;
 #else
     FreeListBuilder<false> free_queue;
@@ -149,7 +149,7 @@ namespace snmalloc
       auto p = tmp_fl.take(key);
       fast_free_list = tmp_fl;
 
-#ifdef CHECK_CLIENT
+#ifdef SNMALLOC_CHECK_CLIENT
       entropy.refresh_bits();
 #else
       UNUSED(entropy);