Fix external pagemap usage. (#221)

At some point in the refactoring, these were broken.

src/mem/chunkmap.h

@@ -138,26 +138,23 @@ namespace snmalloc
     inline static ChunkmapPagemap* external_pagemap;
 
   public:
-    /**
-     * Constructor.  Accesses the pagemap via the C ABI accessor and casts it to
-     * the expected type, failing in cases of ABI mismatch.
-     */
-    ExternalGlobalPagemap()
-    {
-      const snmalloc::PagemapConfig* c;
-      external_pagemap =
-        ChunkmapPagemap::cast_to_pagemap(snmalloc_pagemap_global_get(&c), c);
-      if (!external_pagemap)
-      {
-        Pal::error("Incorrect ABI of global pagemap.");
-      }
-    }
-
     /**
      * Returns the exported pagemap.
+     * Accesses the pagemap via the C ABI accessor and casts it to
+     * the expected type, failing in cases of ABI mismatch.
      */
     static ChunkmapPagemap& pagemap()
     {
+      if (external_pagemap == nullptr)
+      {
+        const snmalloc::PagemapConfig* c = nullptr;
+        void* raw_pagemap = snmalloc_pagemap_global_get(&c);
+        external_pagemap = ChunkmapPagemap::cast_to_pagemap(raw_pagemap, c);
+        if (!external_pagemap)
+        {
+          Pal::error("Incorrect ABI of global pagemap.");
+        }
+      }
       return *external_pagemap;
     }
   };

src/mem/pagemap.h

@@ -395,7 +395,7 @@ namespace snmalloc
      */
     size_t index_for_address(uintptr_t p)
     {
-      return bits::align_down(static_cast<size_t>(p) >> SHIFT, OS_PAGE_SIZE);
+      return (static_cast<size_t>(p) >> SHIFT) % OS_PAGE_SIZE;
     }
 
     /**