kmodtool: fix preun script

Prior to this commit, I could not uninstall the RPMs with yum, because
the %preun script was failing, because %{kmod_version} and
%{kmod_release} were undefined. The only way to uninstall the RPM was
"rpm -e kmod-ceph --noscripts".

The ELRepo 6 kmodtool templates use %{version} and %{release} here. That
solves the problem.

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>