ocfs2: document access rules for blocked_lock_list

ocfs2_super->blocked_lock_list and ocfs2_super->blocked_lock_count have some
usage restrictions which aren't immediately obvious to anyone reading the
code. It's a good idea to document this so that we avoid making costly
mistakes in the future.
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

fs/ocfs2/ocfs2.h

@@ -262,6 +262,12 @@ struct ocfs2_super
 	unsigned long dc_wake_sequence;
 	unsigned long dc_work_sequence;
 
+	/*
+	 * Any thread can add locks to the list, but the downconvert
+	 * thread is the only one allowed to remove locks. Any change
+	 * to this rule requires updating
+	 * ocfs2_downconvert_thread_do_work().
+	 */
 	struct list_head blocked_lock_list;
 	unsigned long blocked_lock_count;