[RFC] vfs generic subtree support

[Dmitry Monakhov <dmonakhov@openvz.org>]

Initially i've posted RFC patch-set which add subtree support for ext4.
http://marc.info/?l=linux-ext4&m=126563931215496&w=2
But in fact this is rather generic feature which may be implemented 
in vfs layer similar to namespace or security feature.

A subtree of a directory tree T is a tree consisting of a directory
(the subtree root) in T and all of its descendants in T.

Subtree assumptions:
* Each inode has subtree id (this id is stored inside inode).
* Subtree id is inherent from parent directory if corresponding flag is set
* Inode can not belongs to different subtrees

i've ommit subtree_type feature in order to simplify brief explanation.

Subtree with id == 0 has special meaning. It may contains roots of
other subtrees (this feature is used for cross subtree renames)
This feature has much in common with XFS project_id.

There are many usecases for such subtrees
1) Choroot environment or Containers on common file-system
   administrator creates a subtree,and setup quota
   #mkdir chroot-env
   #chattr -Q ${subtree_id} chroot-env
   #tar jxf /tmp/fedora-x86_64.tar.bz2 -C chroot-env
   #### as soon as subtree id inherented from parent untared content
   #### automatically belongs to ${subtree_id}
   #quotactl --type=subtree --id=${subtree_id} --bsoft=1000000 --bhard=1000000
   ###
   #chroot chroot-env /bin/bash
2) NFS quota on server side
   Administrator perform exactly the same stages, but instead of chroot
   hi just export result subtree to nfs server
One may imagine it's own usecase.


Usually we already have some content which we want to see as subtree
In this case we have to in-depth traverse it from the subtree root.
add_to_subtree(dir, subtree_id) /* in-depth  tree traversal */
{
	while(de = getdents(dir)) {
	     if (IS_DIR(de))
		    add_to_subtree(de, subtree_id)
             else
	            set_subtree_id(de, subtree);
       }
       set_subtree_id(de, subtree);
}
It is possible to manipulate subtree content

For example we have following hierarchy
/root/subtree-1/a/b/c/d/e/f/g
/root/subtree-2/dir/
want rename /root/subtree-1/a to /root/subtree-2/dir/AA
first we have to move "a" to default tree "root"
 #rename /root/subtree-1/a /root/a-1
Then we have assign to default subtree. Do it in width traverse order
Because otherwise this result in subtree assumptions violation.
 #walk_in_width(/root/AA, 0 /* default subtree */)
Then  assign target subtree_id
 #walk_in_depth(/root/AA, 2)
Ok now all content from /root/a-1 belongs to subtree_id == 2 so it is possible
rename it in to target place
 #rename /root/a-1 /root/subtree-2/dir/AA