kernel code reading: kernfs child dirs
kernel code test: rb_tree iteration
kernel code test: refine the dump of rb_tree

change the order to make it looks the same when switch the window.
kernel code reading: kernfs retrieve inode
kernel code reading: how pwd get the working directory
kernel code test: refine output of request_region_test()
kernel code reading: how ls works
kernel code reading: connetion between kernfs_node and inode/dentry
kernel code reading: add a flag to case list_for_each_entry_xxx()

If there is no flag, each time we invoke these function will re-init the whole
list. While actually, we just want to dump the list content.

This one fix this issue by adding a flag. When it is 1, it will re-init the
list. When it is 0, it just dump the list.
kernel code test: netdevice_notifier