vlserver: Use correct literal in bounds check

The base array has VL_MAX_ADDREXTBLKS elements. Use this when
checking for an array overflow, rather than VL_ADDREXTBLK_SIZE.

Caught by coverity (#985600)

Reviewed-on: http://gerrit.openafs.org/9346
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 6a54bf735871d93bc77e5cf166e506f4f2423d0a)

Change-Id: I056432a1bf85b175ee4fc307d0662d072724b272
Reviewed-on: http://gerrit.openafs.org/11034
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>