vlserver: unsigned values are always >0

Maxvolidbump is defined as an unisgned int, and thus can never be less
than 0. Remove the pointless check, as it just makes clang sad.

Reviewed-on: http://gerrit.openafs.org/7084
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 350b5d58779cc127b96497283c2d982e099b911c)

Change-Id: Ib902a5b165219469db5f370ebea4c078f10726ee
Reviewed-on: http://gerrit.openafs.org/10740
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>