tmpfiles: move validation/normalization of path before we use it

tmpfiles: move validation/normalization of path before we use it

We need to normalize/validate the path (i.e. first column) of
tmpfiles.d/ lines before we start using the path, otherwise we'll use it
before it's known to be good. This matters since for some line types the
path is mangled into the argument column (i.e. sevents column), and we
should only do that once we know it's in a good state.