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When aufs meets a new dir inode on a branch in lookup, it tests whether
the inode is in the list which the branch has. If the inode is found, it
means the dir has ever been logically renamed and there is some info
about the name under that dir. Then aufs tries loading the info, and
continues looking up using the before-renamed name on the lower
branches.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

When DIRREN is enabled and activated, the error case where
aufs rename(2) used to return EXDEV will be gone.
Aufs rename(2) registers the renaming dir inum to the list in the
branch, creates the detailed info file, and returns a success.

If udba=notify option is specified with dirren, the internal detection
may not work correctly since aufs may not be able to find the target
name.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The detailed info per renamed directory is stored in a regular file per
branch, ie. when each of two lower branches contains the same named
entry, then the created info files will be two.
The file is created internally by aufs rename(2) and loaded by lookup.
Also when the actual rename on the branch fails, the newly created or
stored info file should be all reverted.

When the renamed dir is renamed-back to the previous/original name, then
the info file has to be removed.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

DIRREN gives an identifier to every branch, and it is used as a part of
the filename of the detailed info file.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This commits brings a list of the inode numbers which indicates the
logically renamed dir into a branch. The list will be referred in
lookup, and its lifetime is equivalent to the branch's, ie. the list is
loaded/created in adding a branch, and stored/deleted in deleting a
branch. The simple storing happens in remounting and unmounting aufs
too.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>