chmod, chown: -R --preserve-root judges the operand by a separate path lookup

[abendrothj]

With -R -H or -L, chmod/chown/chgrp check --preserve-root on one lookup of the operand, then change it and descend through another. If the operand is replaced in between, for example by a symlink to /, the check sees the old directory and the recursion runs on /. Separately, chmod -R -P re-resolves the operand to change its mode, so a symlink put at that name in between is followed.

To reproduce, run chmod -R -H -v --preserve-root u+r d repeatedly while another process keeps swapping d with a symlink to / (renameat2 with RENAME_EXCHANGE). Some runs list files under /.