Committed, thanks for the comments.
Note to myself (and others that might be interested in the O(1)) version):
we can't simply call openat(dirfd, "..", O_RDONLY) to re-open the current directory's file descriptor after having walked a into one of its subdirectories because if this subdirectory is actually a link, we'll open the parent directory of the target directory, instead of the current (toppath) directory. OTOH, if the user passes followlinks=True, then we don't have to bother with openat() and friends in which case we don't have to bother passing FDs between calls to fwalk().