OpenZFS 8794 - cstyle generates warnings with recent perl

Authored by: Dominik Hassler <hadfl@omniosce.org>
Reviewed by: Andy Fiddaman <andy@omniosce.org>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/8794
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/578f67364c
Closes #6973