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Aufs allows users' direct branch access (UDBA), ie. by-passing aufs.
Of course it will make aufs inode/dentry-caches obsolete. In order to
detect such change by the several tests, "udba=" option is introduced.

For details, see lookup.txt in later commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>