MDEV-254: Server hang with FLUSH TABLES WITH READ LOCK AND DISABLE CHECKPOINT

The code to re-enable checkpointing after UNLOCK TABLES was lost in the 5.5
merge, so re-add it back in.

mysql-test/r/flush-innodb.result

+FLUSH TABLES WITH READ LOCK AND DISABLE CHECKPOINT;
+UNLOCK TABLES;
+CREATE TABLE t1 ( m MEDIUMTEXT ) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ( REPEAT('i',1048576) );
+DROP TABLE t1;

mysql-test/t/flush-innodb.test

+--source include/have_innodb.inc
+
+# MDEV-254: Server hang with FLUSH TABLES WITH READ LOCK AND DISABLE CHECKPOINT
+FLUSH TABLES WITH READ LOCK AND DISABLE CHECKPOINT;
+UNLOCK TABLES;
+CREATE TABLE t1 ( m MEDIUMTEXT ) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ( REPEAT('i',1048576) );
+
+DROP TABLE t1;

sql/lock.cc

@@ -1041,6 +1041,15 @@ void Global_read_lock::unlock_global_read_lock(THD *thd)
 
   DBUG_ASSERT(m_mdl_global_shared_lock && m_state);
 
+  if (thd->global_disable_checkpoint)
+  {
+    thd->global_disable_checkpoint= 0;
+    if (!--global_disable_checkpoint)
+    {
+      ha_checkpoint_state(0);                   // Enable checkpoints
+    }
+  }
+
   if (m_mdl_blocks_commits_lock)
   {
     thd->mdl_context.release_lock(m_mdl_blocks_commits_lock);