MDEV-18154 Deadlock and assertion upon no-op ALTER under LOCK TABLES

1. Revert incorrect treatment of m_needs_reopen;
2. Close single instance of TABLE instead of all instances since
   reopened only those that are marked for reopen.

sql/sql_base.cc

@@ -2488,9 +2488,16 @@ Locked_tables_list::reopen_tables(THD *thd, bool need_reopen)
     {
       if (!table_list->table || !table_list->table->needs_reopen())
         continue;
-      /* no need to remove the table from the TDC here, thus (TABLE*)1 */
-      close_all_tables_for_name(thd, table_list->table->s,
-                                HA_EXTRA_NOT_USED, (TABLE*)1);
+      for (TABLE **prev= &thd->open_tables; *prev; prev= &(*prev)->next)
+      {
+        if (*prev == table_list->table)
+        {
+          thd->locked_tables_list.unlink_from_list(thd, table_list, false);
+          mysql_lock_remove(thd, thd->lock, *prev);
+          close_thread_table(thd, prev);
+          break;
+        }
+      }
       DBUG_ASSERT(table_list->table == NULL);
     }
     else

sql/sql_table.cc

@@ -7831,7 +7831,6 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   Create_field *def;
   Field **f_ptr,*field;
   MY_BITMAP *dropped_fields= NULL; // if it's NULL - no dropped fields
-  bool save_reopen= table->m_needs_reopen;
   DBUG_ENTER("mysql_prepare_alter_table");
 
   /*
@@ -8511,9 +8510,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   alter_info->create_list.swap(new_create_list);
   alter_info->key_list.swap(new_key_list);
   alter_info->check_constraint_list.swap(new_constraint_list);
-  DBUG_RETURN(rc);
 err:
-  table->m_needs_reopen= save_reopen;
   DBUG_RETURN(rc);
 }