Fix external pagemap usage. (#221)

At some point in the refactoring, these were broken.

src/test/func/external_pagemap/external_pagemap.cc

@@ -0,0 +1,30 @@
+#if defined(SNMALLOC_PASS_THROUGH) || defined(_WIN32)
+// This test does not make sense with malloc pass-through, skip it.
+// The malloc definitions are also currently incompatible with Windows headers
+// so skip this test on Windows as well.
+int main()
+{
+  return 0;
+}
+#else
+#  define SNMALLOC_EXPOSE_PAGEMAP 1
+#  include <override/malloc.cc>
+
+int main()
+{
+  auto& p = ExternalGlobalPagemap::pagemap();
+  auto& global = GlobalPagemap::pagemap();
+  SNMALLOC_CHECK(&p == &global);
+  // Get a valid heap address
+  uintptr_t addr = reinterpret_cast<uintptr_t>(malloc(42));
+  // Make this very strongly aligned
+  addr &= ~0xfffffULL;
+  void* page = p.page_for_address(addr);
+  SNMALLOC_CHECK(page == p.page_for_address(addr + 128));
+  size_t idx = p.index_for_address(addr);
+  size_t idx2 = p.index_for_address(addr + SUPERSLAB_SIZE);
+  // If the pagemap ends up storing things that are not uint8_t, this test
+  // will need modifying.
+  SNMALLOC_CHECK(idx2 = ((idx + 1) % OS_PAGE_SIZE));
+}
+#endif