online alter: relax the lock to upgrade to on the last replication stage

This will allow selects pass until the rename/commit stage.
The lock is anyway upgraded to MDL_EXCLUSIVE later by the
wait_while_table_is_used call in mysql_alter_table.

sql/sql_table.cc

@@ -12354,7 +12354,7 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
     DEBUG_SYNC(thd, "alter_table_online_before_lock");
 
     int lock_error=
-        thd->mdl_context.upgrade_shared_lock(from->mdl_ticket, MDL_EXCLUSIVE,
+        thd->mdl_context.upgrade_shared_lock(from->mdl_ticket, MDL_SHARED_NO_WRITE,
                                      (double)thd->variables.lock_wait_timeout);
     if (!error)
       error= lock_error;