Fix missing dkms modules after upgrades

Fix missing dkms modules after upgrades

If you were upgrading from say, fc28->fc29, on ZFS version X, the RPMs
macros would get called like this:

%post X.fc29
   - This is the step where fc29 gets built by dkms.
     As part of the build, dkms automatically removes the previous
     modules before building the new ones.  It then builds the new
     modules.
%preun X.fc28
   - Right before this step, X.fc29 is be built and installed, but
     since it has the same X, it's files get inadvertently removed
     by fc28's uninstall.
%postun X.fc28

This patch updates %preun X.fc28 to see if we're upgrading or
uninstalling.  If we're uninstalling, then remove our files. If we're
upgrading then do nothing, since will know dkms will have already
removed our files in %post X.fc29.

Note that since this fixes the %preun step, it's effect isn't going
to be noticed immediately.  It will only be seen when packages
with this fix are upgraded to a newer version.

Reviewed-by: Ralf Ertzinger <ralf@skytale.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #6902
Closes #8216