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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>