Promote _make_hd_dn() to make_hd_dn() and get rid of the wrapper.

Promote _make_hd_dn() to make_hd_dn() and get rid of the wrapper.

The wrapper is just hiding what the code's doing, and all the other code
around where we use it does the same thing anyway.  This hopefully
convinces coverity we're not really dereferencing "buf" there unless
it's nonzero.

Signed-off-by: Peter Jones <pjones@redhat.com>