6551 cmd/zpool: cleanup gcc warnings

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Robert Mustacchi <rm@joyent.com>

illumos/illumos-gate@b327cd3f3b4dab4f29e7140159b1e01ed2ceef2a