Bug#986374: zfs-zed: broken purge detection in postrm script

[Chris Hofstaedtler]

* наб <nabijaczlew...@nabijaczleweli.xyz> [210404 20:54]:
> /var/lib/dpkg/info/zfs-zed.postrm: 7: [: upgrade: unexpected operator

Apparently introduced in the fix for #983401.

Chris