Fixed wrong assignment in calculate_block_sizes() for MEM_ROOT

The effect was that that ROOT_FLAG_THREAD_SPECIFIC was cleared and
the memory allocated by memroot would be contributed the the system,
not to the thread.

This exposed a bug in how "show explain for ..." allocated data.
- The thread that did provide the explain allocated data in the
  "show explain" threads mem_root, which is marked as THREAD_SPECIFIC.
- Fixed by allocating the explain data in a temporary explain_mem_root
  which is not THREAD_SPECIFIC.

Other things:
- Added extra checks when using update_malloc_size()
- Do not call update_malloc_size() for memory not registered with
  update_malloc_size(). This avoid some wrong 'memory not freed' reports.
- Added a checking of 'thd->killed' to ensure that
  main.truncate_notembedded.test still works.

Reported by: Yury Chaikou

sql/mysqld.cc

@@ -280,6 +280,12 @@ extern "C" sig_handler handle_fatal_signal(int sig);
 
 int init_io_cache_encryption();
 
+extern "C"
+{
+  static void my_malloc_size_cb_func(long long size,
+                                     my_bool is_thread_specific);
+}
+
 /* Constants */
 
 #include <welcome_copyright_notice.h> // ORACLE_WELCOME_COPYRIGHT_NOTICE