BUG#28838 - duplicate external_lock in mysql_alter_table

Removed duplicate call to handler::external_lock() when
ALTER TABLE that doesn't need to copy a table (quick
ALTER TABLE) was executed.

Also quick ALTER TABLE doesn't hold LOCK_open anymore when
it enables/disables indexes.

sql/sql_table.cc

@@ -3789,11 +3789,9 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   {
     VOID(pthread_mutex_lock(&LOCK_open));
     wait_while_table_is_used(thd, table, HA_EXTRA_FORCE_REOPEN);
-    table->file->external_lock(thd, F_WRLCK);
+    VOID(pthread_mutex_unlock(&LOCK_open));
     alter_table_manage_keys(table, table->file->indexes_are_disabled(),
                             alter_info->keys_onoff);
-    table->file->external_lock(thd, F_UNLCK);
-    VOID(pthread_mutex_unlock(&LOCK_open));
     error= ha_commit_stmt(thd);
     if (ha_commit(thd))
       error= 1;