fs-util: when calling chase_symlinks() with root path, leave root part unresolved

fs-util: when calling chase_symlinks() with root path, leave root part unresolved

Previously there was a weird asymmetry: initially we'd resolve the
specified prefix path when chasing symlinks together with the actual
path we were supposed to cover, except when we hit an absolute symlink
where we'd use the root as it was. Let's unify handling here: the prefix
path is never resolved, and always left as it is.

This in particular fixes issues with symlinks in the prefix path, as
that confused the check that made sure we never left the root directory.

Fixes: #14634
Replaces: #14635