omfs: rename() needs to mark old_inode dirty after ctime update

we *do* mark it dirty before, but it doesn't guarantee that we
don't get preempted just before assignment to ->i_ctime, with
inode getting written out before we get CPU back...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Bob Copeland <me@bobcopeland.com>