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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>

Basically copy-up is done by these steps using au_pin (in another
commit).
- lock the target parent mutex
- lookup a negative dentry with a whiteout-ed temporary unique name
- create it
- unlock the target parent mutex
- copy filedata
- copy metadata (inode attributes)
- lock the target parent mutex
- rename the temporary name to the target name
- unlock the target parent mutex

This commit contains step2 mainly.
I hope someday aufs uses O_TMPFILE for this.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The functions to create/delete the opaque directory marker (called
'diropq') on the added writable branch.
For details, refer to previous commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Actually prepare the whiteout bases on the adding writable branch.
For details, refer to previous commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The writable branch prepares a few files and dirs for whiteouts.
For branch filesystems which doesn't support link(2), there is "nolwh"
attribute. On the branch which is specified this attribute, aufs never
try link(2) for whitout and always creat(2) it.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The whiteout represents a logical deletion.
Although the document in this commit mentioned about rmdir(2) and
rename(2) for dir, this commit doesn't contain such functions. They will
be added in later commits.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>