Bug#37132323: db cache is not flushed on DROP USER

The DB grant cache is not cleaned up at DROP USER. This was causing ghost grants until FLUSH PRIVILEGES.

Fixed by flushing the cache.

Change-Id: I1096558daffab2d5f75fe12af723680d5c1d2416

Author: gkodinov

sql/auth/sql_user.cc

@@ -3133,6 +3133,7 @@ bool mysql_drop_user(THD *thd, List<LEX_USER> &list, bool if_exists,
     /* Rebuild 'acl_check_hosts' since 'acl_users' has been modified */
     rebuild_check_host();
     rebuild_cached_acl_users_for_name();
+    clear_and_init_db_cache(); /* Clear privilege cache */
 
     if (result && !thd->is_error()) {
       String operation_str;