Remove the openafs module from DKMS if necessary before adding

Remove the openafs module from DKMS if necessary before adding

* In the postinst of openafs-modules-dkms, if the openafs module is
  already added in DKMS, try to remove it first before adding it.  This
  should more correctly handle the case of a user installing this
  package without the correct kernel headers, having it fail in
  postinst, and then installing the correct headers and having dpkg
  attempt to configure the package again.  Thanks, Philipp Kaluza.
  (Closes: #553542)