MDEV-30978: On slave XA COMMIT/XA ROLLBACK fail to return an error in read-only mode

Where a read-only server permits writes through replication, it
should not permit user connections to commit/rollback XA
transactions prepared via replication. The bug reported in
MDEV-30978 shows that this can happen. This is because there is no
read only check in the XA transaction logic, the most relevant one
occurs in ha_commit_trans() for normal statements/transactions.

This patch extends the XA transaction logic to check the read only
status of the server before performing an XA COMMIT or ROLLBACK.

Reviewed By:
Andrei Elkin <andrei.elkin@mariadb.com>

sql/sql_class.h

@@ -2787,6 +2787,16 @@ public:
     return m_binlog_filter_state;
   }
 
+  /**
+    Checks if a user connection is read-only
+  */
+  inline bool is_read_only_ctx()
+  {
+    return opt_readonly &&
+           !(security_ctx->master_access & PRIV_IGNORE_READ_ONLY) &&
+           !slave_thread;
+  }
+
 private:
   /**
     Indicate if the current statement should be discarded