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Special support for filesystems which acquires an inode mutex at final
closing a file, eg, hfsplus.
This trick is very simple and stupid, just to open the file before really
necessary open to tell hfsplus that this is not the final closing.
The caller should call au_h_open_pre() after acquiring the inode mutex,
and au_h_open_post() after releasing it.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This feature automatically handles MVDOWN in other commits.
In user-space, a daemon monitors the free space of the branch and issues
MVDOWN ioctl automatically when necessary. The main role is in
user-space and several options are implemented.
For a branch to join the FHSM circle, a new attribute 'fhsm' should be
specified.

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

These are very old options.
Since Unionfs created 'diropq' unconditionally in mkdir(2), old users
may expect the same behaviour. But there are cases where 'diropq' is
unnecessary. The aufs default behaviour is to create 'diropq' only when
it is necessary.
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>

This is a feature to optimize for rmdir and rename dir.
When the number of whiteouts under the target dir is very many, it may
take a long time to remove them all. To prevent this, 'dirwh=%d' option
specifies the watermark to decide when to remove them.

For details, see aufs manual in aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement i_op->rename().
This is a big monster and I don't like it.

In this version, the copy-up always happen.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>