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)

Change-Id: Ie60ad5cc79c41245028887871759776549694426
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>