fix: don't call check_section() in filesystem ctor (fixes gh #183)

This regression was introduced in the v0.7.3 release and caused each
section header to be read from the filesystem image at mount time,
defeating the purpose of the section index.

src/dwarfs/filesystem_v2.cpp

@@ -524,11 +524,17 @@ filesystem_<LoggerPolicy>::filesystem_(
   section_map sections;
 
   while (auto s = parser.next_section()) {
-    check_section(*s);
-
     if (s->type() == section_type::BLOCK) {
+      // Don't use check_section() here because it'll trigger the lazy
+      // section to load, defeating the purpose of the section index.
+      // See github issue #183.
+      LOG_DEBUG << "section " << s->name() << " @ " << s->start() << " ["
+                << s->length() << " bytes]";
+
       cache.insert(*s);
     } else {
+      check_section(*s);
+
       if (!s->check_fast(*mm_)) {
         DWARFS_THROW(runtime_error, "checksum error in section: " + s->name());
       }