MDEV-38383 Fix MDEV-38073 MTR test warning

MDEV-38073 MTR test started to fail with a warning after upstream merge
from 11.4 a7528a6190 because THD responsible
for creating SST user became read-only when the server was started with
--transaction-read-only=TRUE.
make sure the readonly flag on THDs created for wsp::thd utility class is
cleared regardless of the --transaction-read-only value as it is intended
only for client-facing THDs.

mysql-test/suite/galera/r/MDEV-38073.result

@@ -1,5 +1,7 @@
 connection node_2;
 connection node_1;
+connection node_1;
+connection node_2;
 connection node_2;
 connection node_1;
 CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;

mysql-test/suite/galera/t/MDEV-38073.test

@@ -7,6 +7,11 @@
 --source include/have_innodb.inc
 --source include/big_test.inc
 
+# Save original auto_increment_offset values.
+--let $node_1=node_1
+--let $node_2=node_2
+--source include/auto_increment_offset_save.inc
+
 # Restart the node 2 in transaction_read_only mode.
 --connection node_2
 --source include/shutdown_mysqld.inc
@@ -37,3 +42,5 @@ SELECT COUNT(*) = 2 FROM t1;
 # Cleanup
 --connection node_1
 DROP TABLE t1;
+
+--source include/auto_increment_offset_restore.inc

sql/wsrep_utils.cc

@@ -508,6 +508,8 @@ thd::thd (my_bool ini, bool system_thread)
     wsrep_assign_from_threadvars(ptr);
     wsrep_store_threadvars(ptr);
 
+    ptr->tx_read_only= false;
+    ptr->variables.tx_read_only= false;
     ptr->variables.tx_isolation= ISO_READ_COMMITTED;
     ptr->variables.sql_log_bin = 0;
     ptr->variables.option_bits &= ~OPTION_BIN_LOG; // disable binlog