OpenZFS 6931 - lib/libzfs: cleanup gcc warnings

Authored by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/6931
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/88f61de
Closes #5741