ocfs2: Rename cleanups

ocfs2_rename() does direct manipulation of the dirent it's gotten back from
a directory search. Wrap this manipulation inside of a function so that we
can transparently change directory update behavior in the future. As an
added bonus, this gets rid of an ugly macro.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Reviewed-by: Joel Becker <joel.becker@oracle.com>