afs/VNOPS: use osi_AllocSmallSpace() correctly

Send the size down to the allocator so it can perform some
consistency checks.

Change-Id: I6c089ab912014c639888b744a92a28813fb4b3c4
Reviewed-on: http://gerrit.openafs.org/11168
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>