ln: ln -s -f fails with ENOTDIR when the destination's parent path is a symlink

[varranvar]

ln -s -f fails with ENOTDIR when the destination's parent path is a symlink. This is a regression in 0.12.0.

$ mkdir -p modules dst                                                                                 
$ ln -s -f "$PWD/modules" dst/ucc    # ok: dst/ucc -> modules                                          
$ ln -s -f "$PWD/modules" dst/ucc    # ok, but creates modules/modules -> modules    
$ ln -s -f "$PWD/modules" dst/ucc    # fails                                                           
ln: failed to create symbolic link 'dst/ucc/modules': Not a directory                                  

Expected (GNU, uutils 0.11.0): The third command succeeds. The forced replacement unlinks and recreates the link.

The cause seems to be replace_link() introduced in #14623. In replace_link(), the initial symlinkat() fails with EEXIST because modules/modules already exists, and the fallback then opens the destination's parent with O_NOFOLLOW | O_DIRECTORY. Since the parent ends in a symlink, the open fails with ENOTDIR.