venus: appease gcc8's -Wformat-string

venus: appease gcc8's -Wformat-string

Interestingly, even before this commit, the buffer size was larger
than what the kernel would accept.  Since the kernel does its own
length checking, it's simplest to just allow slightly larger requests
here and have them fail later.

Reviewed-on: https://gerrit.openafs.org/13471
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit dff81f1b78fecc54f5af91f7d728925ffca62d2c)

Change-Id: Ie19d887abebdd3603a04c06723f5cb750eb654f8
Reviewed-on: https://gerrit.openafs.org/13740
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>