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For details, read the document in this commit.
I don't like this approach, but there is no other way currently. But it
seems that UnionMount is trying add siblings of f_dentry and d_inode for
linux-4.0 or later. It may become another light for aufs too.

The finfo object which has ever mmapped is excluded from
refreshing (based upon fi_mmapped). Otherwise we may corrupt the process
memory space.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement f_op->open() for non-directory.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>