Fix gcc warnings

Remove -Wunused-but-set-variable (error). Cast calls with
(void) to note that the return value is explicitly ignored.

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division