OpenZFS 6550 - cmd/zfs: cleanup gcc warnings

OpenZFS 6550 - cmd/zfs: cleanup gcc warnings

Porting Notes:
- Many of the fixes proposed by this patch were already applied.
In the cases where a different but equivalent fix was made the
code was updated with the OpenZFS version to minimize differences.

Authored by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6550
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c16bcc4
Closes #5591