6550 cmd/zfs: cleanup gcc warnings

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Igor Kozhukhov <ikozhukhov@gmail.com>

illumos/illumos-gate@c16bcc4577f389573eff411c7b7e040294078c3b