MDEV-32269 InnoDB after ALTER TABLE…IMPORT TABLESPACE may not be crash safe

mtr_t::commit(): If IMPORT TABLESPACE is first-time-dirtying blocks,
acquire both log_sys.mutex and log_sys.flush_order_mutex to assign
a valid m_commit_lsn so that the block will be inserted into the
correct position of buf_pool.flush_list.

This fixes occasional debug assertion failures when running the
regression test suite.

Reviewed by: Vladislav Lesin

storage/innobase/mtr/mtr0mtr.cc

@@ -453,10 +453,16 @@ void mtr_t::commit()
     {
       ut_ad(m_log_mode == MTR_LOG_NO_REDO);
       ut_ad(m_log.size() == 0);
-      m_commit_lsn= log_sys.get_lsn();
-      lsns= { m_commit_lsn, PAGE_FLUSH_NO };
       if (UNIV_UNLIKELY(m_made_dirty)) /* This should be IMPORT TABLESPACE */
+      {
+        mysql_mutex_lock(&log_sys.mutex);
+        m_commit_lsn= log_sys.get_lsn();
         mysql_mutex_lock(&log_sys.flush_order_mutex);
+        mysql_mutex_unlock(&log_sys.mutex);
+      }
+      else
+        m_commit_lsn= log_sys.get_lsn();
+      lsns= { m_commit_lsn, PAGE_FLUSH_NO };
     }
 
     if (m_freed_pages)