MDEV-27469: Assertion failure in defragment due to tx_read_only

In commit c5fd9aa5 (MDEV-25919)
we prevented the function dict_stats_save_index_stat()
from being called in read-only mode in dict_stats_save(),
but not elsewhere.

dict_stats_save_defrag_summary(), dict_stats_save_defrag_stats():
If the transaction is in read-only mode, return DB_READ_ONLY
and do not attempt to lock or modify anything.

storage/innobase/dict/dict0defrag_bg.cc

 /*****************************************************************************
 
-Copyright (c) 2016, 2021, MariaDB Corporation.
+Copyright (c) 2016, 2022, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -253,7 +253,9 @@ dberr_t dict_stats_save_defrag_summary(dict_index_t *index, THD *thd)
   trx_t *trx= trx_create();
   trx->mysql_thd= thd;
   trx_start_internal(trx);
-  dberr_t ret= lock_table_for_trx(table_stats, trx, LOCK_X);
+  dberr_t ret= trx->read_only
+    ? DB_READ_ONLY
+    : lock_table_for_trx(table_stats, trx, LOCK_X);
   if (ret == DB_SUCCESS)
     ret= lock_table_for_trx(index_stats, trx, LOCK_X);
   row_mysql_lock_data_dictionary(trx);
@@ -388,7 +390,9 @@ dict_stats_save_defrag_stats(
   trx_t *trx= trx_create();
   trx->mysql_thd= thd;
   trx_start_internal(trx);
-  dberr_t ret= lock_table_for_trx(table_stats, trx, LOCK_X);
+  dberr_t ret= trx->read_only
+    ? DB_READ_ONLY
+    : lock_table_for_trx(table_stats, trx, LOCK_X);
   if (ret == DB_SUCCESS)
     ret= lock_table_for_trx(index_stats, trx, LOCK_X);