dumpon: fix set but not used variable

Signed-off-by: inkeliz <inkeliz@inkeliz.com>

Reviewed by: markj
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1178

(cherry picked from commit 464b1ab216d5f78054626524addf99f5c622da96)