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 <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #6108 

0 files changed, 0 insertions, 0 deletions