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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tony Hutter <[email protected]>
Closes openzfs#6902 
Closes openzfs#8216

rpm/generic/zfs-dkms.spec.in

@@ -72,6 +72,24 @@ echo -e "support or upgrade DKMS to a more current version."
 exit 1
 
 %preun
+# Are we doing an upgrade?
+if [ $1 -ne 0 ] ; then
+	# Yes we are.  Are we upgrading to a new ZFS version?
+	NEWEST_VER=$(dkms status zfs | sed 's/,//g' | sort -r -V | awk '/installed/{print $2; exit}')
+	if [ "$NEWEST_VER" != "%{version}" ] ; then
+		# Yes, it's a new ZFS version.  We'll uninstall the old module
+		# later on in this script.
+		true
+	else
+		# No, it's probably an upgrade of the same ZFS version
+		# to a new distro (zfs-dkms-0.7.12.fc28->zfs-dkms-0.7.12.fc29).
+		# Don't remove our modules, since the rebuild for the new
+		# distro will automatically delete the old modules.
+		exit 0
+	fi
+fi
+
+# If we're here then we're doing an uninstall (not upgrade).
 CONFIG_H="/var/lib/dkms/%{module}/%{version}/*/*/%{module}_config.h"
 SPEC_META_ALIAS="@PACKAGE@-@VERSION@-@RELEASE@"
 DKMS_META_ALIAS=`cat $CONFIG_H 2>/dev/null |