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->atomic_open() is another monster (the other is ->rename() of course).
It operates look-up, create, and open in a single method.
Strictly speaking the behaviour is not atomic from the branch fs's point
of view, while the atomicity is kept in aufs's. This is a second-best
approach. For details, refer to the design document in previous commit.

A simple list 'si_aopen' can be put into aufs inode object, but I don't
think it a good idea because the number of inodes is much larger than
the number of super_block. If I put it into inode, it can be an
un-interesting memory pressure I am afraid.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>