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Implement i_op->rmdir() with supporting logical deletion by whiteout
including all children.

As struct.txt in previous commit described, the target dir is renamed to
a whiteout-ed temporary unique name in rmdir(2), and then removed
asynchronously by the system global workqueue.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

It is hard to implement readdir(3) for aufs virtual directory.
It considers the every whiteout in a single direcotry, as well as the
(first) opaque marker (diropq).
This implementation consumes memory a lot, and I'd suggest you to try
RDU (readdir in userspace) in later commit.

See also the document in previous commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This commit is just to prepare for the succeeding commit, and split to
suppress the size of a single commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Copy the inode attributes between branches.
See also the document in this commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>