MDEV-15063: InnoDB assertion failure !is_owned() at dict0defrag_bg.cc:327

Probem was that dict_sys mutex was owned when calling function
dict_stats_save_defrag_stats() that assumes we do not own
dict_sys mutex.

storage/innobase/dict/dict0defrag_bg.cc

@@ -240,6 +240,7 @@ dict_stats_process_entry_from_defrag_pool()
 		return;
 	}
 
+	mutex_exit(&dict_sys->mutex);
 	dict_stats_save_defrag_stats(index);
 	dict_table_close(table, FALSE, FALSE);
 }