Fix zfs .deb package warning in prerm script

Debian zfs package generated by alien doesn't call the prerm script
(rpm's %preun) with an integer as first parameter, which results in
the following warning:

   "zfs.prerm: line 2: [: remove: integer expression expected"

Modify the if-condition to avoid the warning.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6108